Fiber communication systems and methods

ABSTRACT

An injection locked transmitter for an optical communication network includes a master seed laser source input substantially confined to a single longitudinal mode, an input data stream, and a laser injected modulator including at least one slave laser having a resonator frequency that is injection locked to a frequency of the single longitudinal mode of the master seed laser source. The laser injected modulator is configured to receive the master seed laser source input and the input data stream, and output a laser modulated data stream.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. application Ser. No.16/460,964, filed on Jul. 2, 2019. U.S. application Ser. No. 16/460,964is a continuation in part of U.S. application Ser. No. 16/408,285, filedon May 9, 2019. U.S. application Ser. No. 16/408,285 is a continuationin part of U.S. application Ser. No. 15/861,303, filed on Jan. 3, 2018,now U.S. Pat. No. 10,447,404, issued Oct. 15, 2019. U.S. applicationSer. No. 15/861,303 is a continuation of U.S. application Ser. No.15/283,632, filed on Oct. 3, 2016, now U.S. Pat. No. 9,912,409, issuedMar. 6, 2018. U.S. application Ser. No. 15/283,632 further claims thebenefit of and priority to U.S. Provisional Patent Application Ser. No.62/321,211, filed Apr. 12, 2016. U.S. application Ser. No. 16/408,285further claims the benefit of and priority to U.S. Provisional PatentApplication Ser. No. 62/669,035, filed May 9, 2018, to U.S. ProvisionalPatent Application Ser. No. 62/671,270, filed May 14, 2018, and to U.S.Provisional Patent Application Ser. No. 62/678,110, filed May 30, 2018.U.S. application Ser. No. 16/460,964 further claims the benefit of andpriority to U.S. Provisional Patent Application Ser. No. 62/693,035,filed Jul. 2, 2018. The present application further claims the benefitof and priority to U.S. Provisional Patent Application Ser. No.62/744,498, filed Oct. 11, 2018, and to U.S. Provisional PatentApplication Ser. No. 62/785,016, filed Dec. 26, 2018. All of theseapplications are incorporated herein by reference in their entireties.

BACKGROUND

The field of the disclosure relates generally to fiber communicationnetworks, and more particularly, to optical networks utilizingwavelength division multiplexing.

Telecommunications networks include an access network through which enduser subscribers connect to a service provider. Bandwidth requirementsfor delivering high-speed data and video services through the accessnetwork are rapidly increasing to meet growing consumer demands. Atpresent, data delivery over the access network is growing bygigabits(Gb)/second for residential subscribers, and by multi-Gb/s forbusiness subscribers. Present access networks are based on passiveoptical network (PON) access technologies, which have become thedominant system architecture to meet the growing high capacity demandfrom end users.

Gigabit PON and Ethernet PON architectures are conventionally known, andpresently provide about 2.5 Gb/s data rates for downstream transmissionand 1.25 Gb/s for upstream transmission (half of the downstream rate).10 Gb/s PON (XG-PON or IEEE 10G-EPON) has begun to be implemented forhigh-bandwidth applications, and a 40 Gb/s PON scheme, which is based ontime and wavelength division multiplexing (TWDM and WDM) has recentlybeen standardized. A growing need therefore exists to develophigher/faster data rates per-subscriber to meet future bandwidth demand,and also increase the coverage for services and applications, but whilealso minimizing the capital expenditures (CAPEX) and operationalexpenditures (OPEX) necessary to deliver higher capacity and performanceaccess networks.

One known solution to increase the capacity of a PON is the use of WDMtechnology to send a dedicated wavelength signal to end users. Currentdetection scheme WDM technology, however, is limited by its low receiversensitivity, and also by the few options available to upgrade and scalethe technology, particularly with regard to use in conjunction with thelower-quality legacy fiber environment. The legacy fiber environmentrequires operators to squeeze more capacity out of the existing fiberinfrastructure to avoid costs associated with having to retrench newfiber installment. Conventional access networks typically include sixfibers per node, servicing as many as 500 end users, such as homesubscribers. Conventional nodes cannot be split further and do nottypically contain spare (unused) fibers, and thus there is a need toutilize the limited fiber availability in a more efficient andcost-effective manner.

Coherent technology has been proposed as one solution to increase bothreceiver sensitivity and overall capacity for WDM-PON optical accessnetworks, in both brown and green field deployments. Coherent technologyoffers superior receiver sensitivity and extended power budget, and highfrequency selectivity that provides closely-spaced dense or ultra-denseWDM without the need for narrow band optical filters. Moreover, amulti-dimensional recovered signal experienced by coherent technologyprovides additional benefits to compensate for linear transmissionimpairments such as chromatic dispersion (CD) and polarization-modedispersion (PMD), and to efficiently utilize spectral resources tobenefit future network upgrades through the use of multi-level advancedmodulation formats. Long distance transmission using coherenttechnology, however, requires elaborate post-processing, includingsignal equalizations and carrier recovery, to adjust for impairmentsexperienced along the transmission pathway, thereby presentingsignificant challenges by significantly increasing system complexity.

Coherent technology in longhaul optical systems typically requiressignificant use of high quality discrete photonic and electroniccomponents, such as digital-to-analog converters (DAC),analog-to-digital converters (ADC), and digital signal processing (DSP)circuitry such as an application-specific integrated circuit (ASIC)utilizing CMOS technology, to compensate for noise, frequency drift, andother factors affecting the transmitted channel signals over the longdistance optical transmission. Coherent pluggable modules for metrosolution have gone through C Form-factor pluggable (CFP) to CFP2 andfuture CFP4 via multi-source agreement (MSA) standardization to reducetheir footprint, to lower costs, and also to lower power dissipation.However, these modules still require significant engineering complexity,expense, size, and power to operate, and therefore have not beenefficient or practical to implement in access applications.

Furthermore, according to Nielsen's Law, if current trends continue,high-end end users are expected to require as much as 10 Gb/s by 2023,and 100 Gb/s by 2029. For the new and upcoming generations ofcommunication systems performing under these requirements, the dataspeed will also need to be matched for upstream communications. However,conventional PONs seeking to approach the 100 Gb/s aggregating data ratesuffer from several limitations, due to the reliance on traditionaldirect detection techniques, that render 100 Gb/s technically andeconomically infeasible for these PONs. The conventional directdetection PONs, for example, are known to have poor receiversensitivity, to experience power fading due to chromatic dispersion athigh symbol rates and long transmission distances, and to utilizebandwidth- and power-inefficient modulation. Besides frequencyselectivity and linear detection, Coherent for PONs demonstratessuperior receiver sensitivity, which can be translated to extend reachand split ratio.

In the downlink of conventional PONs, the complexity limits on thetransceiver in an optical line terminal (OLT) at the headend, centraloffice, and/or hub are less stringent than the limits placed on areceiver in an optical network unit (ONU), since the cost of the OLTtransceiver, which sends and receives data to and from multiple ONUs, isshared by all end users supported in the respective network. Incontrast, the cost of each ONU is born solely by the respective enduser. Accordingly, lower costs and lower complexities will moresignificantly impact the ONU than the OLT. For this reason, thecomplexity and high cost of conventional coherent transceivers has beenlimited to point to point (P2P) applications, but prevented fromimplementation in point to multipoint (P2MP) PON applications. That is,despite the significant advantages offered by digital coherenttechnology, the complexity and high cost of conventional coherenttransceivers has not been economically feasible for individual ONUs atthe home location of each subscriber end-user.

P2P and P2MP applications differ in that they P2P connection provides alink between one transmitter and one receiver, whereas a P2MPapplication provides a link between one transmitter and multiplereceivers. Accordingly, in the coherent paradigm, only two coherenttransceivers may be needed in a P2P link, whereas the number of coherenttransceivers needed in the P2MP link (i.e., one coherent transceiver foreach ONU) may be significant (as many as 500, in the example above).

Therefore, the laser source is of critical importance for therealization of such coherent optical transmission systems. That is, onetype of laser may not simply be substituted for another type withoutsignificantly affecting the network. Additionally, the frequency andphase noise of the laser will also significantly affect the performanceof conventional optical coherent transceivers, and impairments therefromhave to be mitigated by carrier-phase recovery (CPR) techniques, sincefrequency and phase noise are directly related to each other, and areclosely related to the linewidth of the laser.

Furthermore, the modulation speed and transmission distance of thenetwork also will strongly depend on the spectral linewidth of thelaser. That is, narrower linewidths are required for higher modulationspeeds (data rates) and longer distance transmissions. Some conventionalcoherent transceivers use an external cavity laser (ECL). From theperformance perspective, ECLs have demonstrated superior performancecapabilities for coherent systems, sufficient for present long haul andmetro distance sensitivity requirements. However, within the accessenvironment, ECLs are considered prohibitively expensive if deployed ateach ONU at an end user's home location. In contrast, Fabry-Perot laserdiodes (FP-LD) and weak-resonant-cavity laser diode (WRC-FPLD) basedtransmitters are considerably less expensive than the costly externallytunable lasers such as ECLs or distributed feedback (DFB)/distributedBragg reflector (DBR) lasers. However, use of these relativelylower-cost, simpler FP lasers is limited by transmission bandwidth andcapacity, particularly in direct-detection systems, and is notapplicable for coherent systems in the conventional use form.

BRIEF SUMMARY

In one aspect, an injection locked transmitter for an opticalcommunication network includes a master seed laser source inputsubstantially confined to a single longitudinal mode, an input datastream, and a laser injected modulator including at least one slavelaser having a resonator frequency that is injection locked to afrequency of the single longitudinal mode of the master seed lasersource. The laser injected modulator is configured to receive the masterseed laser source input and the input data stream, and output a lasermodulated data stream.

In another aspect, an optical network communication system includes, aninput signal source, an optical frequency comb generator configured toreceive the input signal source and output a plurality of phasesynchronized coherent tone pairs. Each of the plurality of phasesynchronized coherent tone pairs includes a first unmodulated signal anda second unmodulated signal. The system further include a firsttransmitter configured to receive the first unmodulated signal of aselected one of the plurality of phase synchronized coherent tone pairsas a seed source and to output a first modulated data stream, and afirst receiver configured to receive the first modulated data streamfrom the first transmitter and receive the second unmodulated signal ofthe selected one of the plurality of phase synchronized coherent tonepairs as a local oscillator source.

In yet another aspect, an optical network communication system includesan optical hub including an optical frequency comb generator configuredto output at least one phase synchronized coherent tone pair having afirst unmodulated signal and a second unmodulated signal, and adownstream transmitter configured to receive the first unmodulatedsignal as a seed source and to output a downstream modulated datastream. The system further includes a fiber node and an end userincluding a downstream receiver configured to receive the downstreammodulated data stream from the downstream transmitter and receive thesecond unmodulated signal as a local oscillator source.

In a still further aspect, a method of optical network processingincludes steps of generating at least one pair of first and secondunmodulated phase synchronized coherent tones, transmitting the firstunmodulated phase synchronized coherent tone to a first transmitter as aseed signal, adhering downstream data, in the first transmitter, to thefirst unmodulated phase synchronized coherent tone to generate a firstmodulated data stream signal, optically multiplexing the first modulateddata stream signal and the second unmodulated phase synchronizedcoherent tone together within a hub optical multiplexer, andcommunicating the multiplexed first modulated data stream signal and thesecond unmodulated phase synchronized coherent tone to a first receiver,by way of fiber optics, for downstream heterodyne detection.

In an aspect, an optical communication network includes an optical hub.The optical hub includes at least one master laser source and at leastone hub transceiver configured to transmit a downstream signal of the atleast one master laser source. The downstream signal includes aplurality of spaced wavelength channels. The network further includes anoptical transport medium configured to carry the downstream signal fromthe optical hub, and a plurality of distributed modem devices operablycoupled to the optical transport medium. The modem is (i) configured toreceive at least one channel of the plurality of spaced wavelengthchannels, and (ii) including at least one child laser source injectionlocked to the master laser source.

In an embodiment, an optical injection locking based coherent opticaltransmitter is provided for a coherent optical communications network.The coherent optical transmitter includes a master laser sourceconfigured to provide a low linewidth frequency channel as a masterlaser signal, and a coherent optical injection locking (COIL) subsystem.The COIL subsystem includes (i) a first slave laser configured for COILwith a master frequency of the master laser signal, (ii) an opticalcirculator configured to inject the master laser signal into a cavity ofthe first slave laser, and (iii) a full-field modulator configured tooutput a first modulated optical signal based on an output of the firstslave laser routed through the optical circulator.

In an embodiment, an optical communication network includes an opticalhub, a receiver, and an optical transport medium. The optical hubincludes a master laser source and a plurality of full-field coherentoptical transmitters disposed proximate the optical hub. Each full-fieldtransmitter of the plurality of full-field coherent optical transmitters(i) is configured to transmit a downstream coherent optical signal usinga center frequency of a master signal from the master laser source, and(ii) includes a first slave laser injection locked to the centerfrequency of the master signal. The optical transport medium operablyconnects the plurality of full-field coherent optical transmitters tothe receiver.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features, aspects, and advantages of the presentdisclosure will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 is a schematic illustration of an exemplary fiber communicationsystem in accordance with an exemplary embodiment of the presentdisclosure.

FIG. 2 is a schematic illustration depicting an exemplary transmitterthat can be utilized with the fiber communication system depicted inFIG. 1.

FIG. 3 is a schematic illustration depicting an alternative transmitterthat can be utilized with the fiber communication system depicted inFIG. 1.

FIG. 4 is a schematic illustration depicting an alternative transmitterthat can be utilized with the fiber communication system depicted inFIG. 1.

FIG. 5 is a schematic illustration depicting an alternative transmitterthat can be utilized with the fiber communication system depicted inFIG. 1.

FIG. 6 is a schematic illustration depicting an exemplary upstreamconnection that can be utilized with the fiber communication systemdepicted in FIG. 1.

FIG. 7 is a schematic illustration depicting an exemplary processingarchitecture implemented with the fiber communication system depicted inFIG. 1.

FIG. 8 is a flow chart diagram of an exemplary downstream opticalnetwork process.

FIG. 9 is a flow chart diagram of an exemplary upstream optical networkprocess that can be implemented with the downstream process depicted inFIG. 8.

FIG. 10 is a schematic illustration of a point to multipoint coherentpassive optical network.

FIG. 11 depicts a comparative distribution between the presentembodiments and conventional systems with respect to relative cost andcomplexity of structural components and algorithms.

FIG. 12A depicts an exemplary use case of an external cavity lasersource.

FIG. 12B depicts an exemplary use case of a Fabry-Perot laser source.

FIG. 12C depicts an exemplary hybrid system for injection locking theexternal cavity laser source depicted in FIG. 12A into the Fabry-Perotlaser source depicted in FIG. 12B.

FIG. 13 is a schematic illustration of an injection locked point tomultipoint coherent passive optical network.

FIG. 14 is a schematic illustration of an exemplary passive opticalnetwork system implementing homodyne detection.

FIG. 15 is a schematic illustration of an exemplary passive opticalnetwork system implementing homodyne detection in a full duplexoperation.

FIG. 16 is a schematic illustration of an exemplary passive opticalnetwork system implementing homodyne detection in a full duplexoperation.

FIG. 17 is a schematic illustration of an exemplary dual-fiber passiveoptical network system implementing homodyne detection.

FIG. 18 depicts an exemplary linewidth acquisition effect for a low-costlaser.

FIG. 19 is a schematic illustration of an exemplary passive opticalnetwork system implementing wavelength division multiplexing.

FIG. 20 is a schematic illustration of an exemplary passive opticalnetwork system implementing heterodyne detection.

FIG. 21 is a schematic illustration of an exemplary passive opticalnetwork system implementing heterodyne detection in a full duplexoperation.

FIG. 22 is a schematic illustration of an alternative passive opticalnetwork system implementing heterodyne detection in a full duplexoperation.

FIG. 23 is a schematic illustration of an exemplary passive opticalnetwork system implementing heterodyne detection in a full duplexoperation.

FIG. 24 is a schematic illustration of an exemplary dual-fiber passiveoptical network system implementing heterodyne detection.

FIG. 25 is a schematic illustration of an exemplary coherent opticalinjection locking transmitter system.

FIG. 26 is a graphical illustration depicting a spectral plot of anoperational multimode Fabry Perot laser.

FIG. 27 is a graphical illustration depicting a spectral plot of acoherent optical injection locked Fabry Perot laser.

FIG. 28 is a graphical illustration depicting a spectral plot of apartially coherent optical injection locked Fabry Perot laser.

FIG. 29 is a graphical illustration depicting a comparative plot ofslave laser frequency against master laser power.

FIG. 30 is a graphical illustration depicting an optimization scheme forinjection locking control.

FIG. 31 is a flow chart diagram of an exemplary control process for thesmart controller depicted in FIG. 25.

FIG. 32 is a schematic illustration of a coherent optical injectionlocked system.

FIG. 33 is a graphical illustration depicting an operational principleof a coherent optical injection locked transmitter implementing I/Qmodulation.

FIGS. 34A-B are graphical illustrations depicting operational principlesof coherent optical injection locked transmitters implementing amplitudemodulation and phase modulation.

FIG. 35 is a schematic illustration of an exemplary single-polarizationcoherent optical injection locked transmitter implementing full-fieldmodulation based on I/Q modulation.

FIG. 36 is a schematic illustration of an alternativesingle-polarization coherent optical injection locked transmitterimplementing full-field modulation based on I/Q modulation.

FIG. 37 is a schematic illustration of an exemplary polarizationmultiplexer transmitter implementing dual-polarization coherent opticalinjection locked full-field modulation.

FIG. 38 is a schematic illustration of an alternative polarizationmultiplexer transmitter implementing dual-polarization coherent opticalinjection locked full-field modulation.

FIG. 39 is a schematic illustration of an exemplary single-polarizationcoherent optical injection locked transmitter implementing full-fieldmodulation based on amplitude and phase modulations.

FIG. 40 is a schematic illustration of an exemplary dual-polarizationcoherent optical injection locked transmitter implementing full-fieldmodulation based on amplitude and phase modulations.

FIG. 41 is a schematic illustration of an exemplary optical network.

FIG. 42 is a schematic illustration of an exemplary optical network.

FIG. 43 is a schematic illustration of an alternative optical network.

FIG. 44 is a schematic illustration of an exemplary optical coherentcommunication system for modulating, detecting, and equalizing coherentsignals in the phase domain.

FIG. 45 is a graphical illustration depicting an operational principle aphase domain mapping module that may be implemented with one or more ofthe embodiments described herein to map a multi-level signal into aphase domain signal.

FIG. 46A is a schematic illustration of an exemplary single-polarizationtransmitter for multi-level signal modulation in the phase domain.

FIG. 46B is a schematic illustration of an alternativesingle-polarization coherent optical injection locking transmitter.

FIG. 47 is a schematic illustration of a phase domain equalizationprocess performed at the receiver-side for post-equalization.

FIG. 48 is a schematic illustration of a phase domain equalizationprocess performed at the transmitter-side for pre-equalization.

FIG. 49 is a flow chart diagram of an exemplary pre-equalization processbased on receiver-side channel estimation.

FIGS. 50A-F are graphical illustrations depicting signal results ofreceiver-side digital signal processing.

FIG. 51 is a graphical illustration depicting a comparative plot of biterror rate against received optical power.

Unless otherwise indicated, the drawings provided herein are meant toillustrate features of embodiments of this disclosure. These featuresare believed to be applicable in a wide variety of systems including oneor more embodiments of this disclosure. As such, the drawings are notmeant to include all conventional features known by those of ordinaryskill in the art to be required for the practice of the embodimentsdisclosed herein.

DETAILED DESCRIPTION

In the following specification and the claims, reference will be made toa number of terms, which shall be defined to have the followingmeanings.

The singular forms “a,” “an,” and “the” include plural references unlessthe context clearly dictates otherwise.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” “approximately,” and “substantially,” are notto be limited to the precise value specified. In at least someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value. Here and throughout thespecification and claims, range limitations may be combined and/orinterchanged; such ranges are identified and include all the sub-rangescontained therein unless context or language indicates otherwise.

As used herein, the terms “processor” and “computer” and related terms,e.g., “processing device”, “computing device”, and “controller” are notlimited to just those integrated circuits referred to in the art as acomputer, but broadly refers to a microcontroller, a microcomputer, aprogrammable logic controller (PLC), an application specific integratedcircuit (ASIC), and other programmable circuits, and these terms areused interchangeably herein. In the embodiments described herein, memorymay include, but is not limited to, a computer-readable medium, such asa random access memory (RAM), and a computer-readable nonvolatilemedium, such as flash memory. Alternatively, a floppy disk, a compactdisc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or adigital versatile disc (DVD) may also be used. Also, in the embodimentsdescribed herein, additional input channels may be, but are not limitedto, computer peripherals associated with an operator interface such as amouse and a keyboard. Alternatively, other computer peripherals may alsobe used that may include, for example, but not be limited to, a scanner.Furthermore, in the exemplary embodiment, additional output channels mayinclude, but not be limited to, an operator interface monitor.

Further, as used herein, the terms “software” and “firmware” areinterchangeable, and include computer program storage in memory forexecution by personal computers, workstations, clients, and servers.

As used herein, the term “non-transitory computer-readable media” isintended to be representative of any tangible computer-based deviceimplemented in any method or technology for short-term and long-termstorage of information, such as, computer-readable instructions, datastructures, program modules and sub-modules, or other data in anydevice. Therefore, the methods described herein may be encoded asexecutable instructions embodied in a tangible, non-transitory, computerreadable medium, including, without limitation, a storage device and amemory device. Such instructions, when executed by a processor, causethe processor to perform at least a portion of the methods describedherein. Moreover, as used herein, the term “non-transitorycomputer-readable media” includes all tangible, computer-readable media,including, without limitation, non-transitory computer storage devices,including, without limitation, volatile and nonvolatile media, andremovable and non-removable media such as a firmware, physical andvirtual storage, CD-ROMs, DVDs, and any other digital source such as anetwork or the Internet, as well as yet to be developed digital means,with the sole exception being a transitory, propagating signal.

Furthermore, as used herein, the term “real-time” refers to at least oneof the time of occurrence of the associated events, the time ofmeasurement and collection of predetermined data, the time for acomputing device (e.g., a processor) to process the data, and the timeof a system response to the events and the environment. In theembodiments described herein, these activities and events occursubstantially instantaneously.

As used herein, “modem termination system” (MTS) refers to a terminationunit including one or more of an Optical Network Terminal (ONT), anoptical line termination (OLT), a network termination unit, a satellitetermination unit, a cable modem termination system (CMTS), and/or othertermination systems which may be individually or collectively referredto as an MTS.

As used herein, “modem” refers to a modem device, including one or morea cable modem (CM), a satellite modem, an optical network unit (ONU), aDSL unit, etc., which may be individually or collectively referred to asmodems.

As described herein, a “PON” generally refers to a passive opticalnetwork or system having components labeled according to known namingconventions of similar elements that are used in conventional PONsystems. For example, an OLT may be implemented at an aggregation point,such as a headend/hub, and multiple ONUs may be disposed and operable ata plurality of end user, customer premises, or subscriber locations.Accordingly, an “uplink transmission” refers to an upstream transmissionfrom an end user to a headend/hub, and a “downlink transmission” refersto a downstream transmission from a headend/hub to the end user, whichmay be presumed to be generally broadcasting continuously (unless in apower saving mode, or the like).

FIG. 1 is a schematic illustration of an exemplary fiber communicationsystem 100 in accordance with an exemplary embodiment of the presentdisclosure. System 100 includes an optical hub 102, a fiber node 104,and an end user 106. Optical hub 102 is, for example, a central office,a communications hub, or an optical line terminal (OLT). In theembodiment shown, fiber node 104 is illustrated for use with a passiveoptical network (PON). End user 106 is a downstream termination unit,which can represent, for example, a customer device, customer premises(e.g., an apartment building), a business user, or an optical networkunit (ONU). In an exemplary embodiment, system 100 utilizes a coherentDense Wavelength Division Multiplexing (DWDM) PON architecture.

Optical hub 102 communicates with fiber node 104 by way of downstreamfiber 108. Optionally, where upstream communication is desired alongsystem 100, optical hub 102 further connects with fiber node 104 by wayof upstream fiber 110. In operation, downstream fiber 108 and upstreamfiber 110 are typically 30 km or shorter. However, according to theembodiments presented herein, greater lengths are contemplated, such asbetween 100 km and 1000 km. In an exemplary embodiment, fiber node 104connects with end user 106 by way of fiber optics 112. Alternatively,fiber node 104 and end user 106 may be integrated as a single device,such as a virtualized cable modem termination system (vCMTS), which maybe located at a customer premises. Where fiber node 104 and end user 106are separate devices, fiber optics 112 typically spans a distance ofapproximately 5000 feet or less.

Optical hub 102 includes an optical frequency comb generator 114, whichis configured to receive a high quality source signal 116 from anexternal laser 118 and thereby generate multiple coherent tones 120(1),120(1′), . . . 120(N), 120(N′). Optical frequency comb generator 114utilizes, for example, a mode-locked laser, a gain-switched laser, orelectro-optic modulation, and is constructed such that multiple coherenttones 120 are generated as simultaneous low-linewidth wavelengthchannels of known and controllable spacing. This advantageous aspect ofthe upstream input signal into system 100 allows a simplifiedarchitecture throughout the entire downstream portion of system 100, asdescribed further below.

Generated coherent tones 120 are fed into an amplifier 122, and theamplified signal therefrom is input into a first hub opticaldemultiplexer 124. In an exemplary embodiment, amplifier 122 is anerbium-doped fiber amplifier (EDFA). Optical hub 102 further includes adownstream transmitter 126 and a hub optical multiplexer 128. In anembodiment, optical hub 102 optionally includes a hub optical splitter130, an upstream receiver 132, and a second hub optical demultiplexer134.

Downstream transmitter 126 includes a downstream optical circulator 136and a downstream modulator 138. In an exemplary embodiment, downstreammodulator 138 is an injection locked laser modulator. Upstream receiver132 includes an upstream integrated coherent receiver (ICR) 140, anupstream analog to digital converter (ADC) 142, and an upstream digitalsignal processor (DSP) 144. In the exemplary embodiment, fiber node 104includes a node optical demultiplexer 146. In an alternative embodiment,where upstream transmission is desired, fiber node 104 further includesa node optical multiplexer 148. In the exemplary embodiment, nodeoptical demultiplexer 146 and node optical multiplexer 148 are passivedevices.

End user 106 further includes a downstream receiver 150. In an exemplaryembodiment, downstream receiver 150 has a similar architecture toupstream receiver 132, and includes a downstream ICR 152, a downstreamADC 154, and a downstream DSP 156. For upstream transmission, end user106 optionally includes end user optical splitter 158, which may belocated within downstream receiver 150 or separately, and an upstreamtransmitter 160. In an exemplary embodiment, upstream transmitter 160has a similar architecture to downstream transmitter 126, and includesan upstream optical circulator 162, and an upstream modulator 164.

In operation, system 100 utilizes optical frequency comb generator 114and amplifier 122 convert the input high quality source signal 116 intomultiple coherent tones 120 (e.g., 32 tones, 64 tones, etc.), which arethen input to first hub optical demultiplexer 124. In an exemplaryembodiment, high quality source signal 116 is of sufficient amplitudeand a narrow bandwidth such that a selected longitudinal mode of signal116 is transmitted into optical frequency comb generator 114 withoutadjacent longitudinal modes, which are suppressed prior to processing bycomb generator 114. First hub optical demultiplexer 124 then outputs aplurality of phase synchronized coherent tone pairs 166(1), 166(2), . .. 166(N). That is, the generated coherent frequency tones 120 areamplified by amplifier 122 to enhance optical power, and thendemultiplexed into multiple separate individual phased synchronizedcoherent tone source pairs 166. For simplicity of discussion, thefollowing description pertains only to coherent tone pair 166(1)corresponding to the synchronized pair signal for the first channeloutput, which includes a first unmodulated signal 168 for Ch1 and asecond unmodulated signal 170 for Ch1′, and their routing through system100.

With source signal 116 of a high quality, narrow band, and substantiallywithin a single longitudinal mode, coherent tone pair 166(1), includingfirst unmodulated signal 168 (Ch1) and second unmodulated signal 170(Ch1′), is output as a high quality, narrowband signal, which thenserves as both a source of seed and local oscillator (LO) signals forboth downstream and upstream transmission and reception directions ofsystem 100. That is, by an exemplary configuration, the architecture ofoptical frequency comb generator 114 advantageously produces highquality continuous wave (CW) signals. Specifically, first unmodulatedsignal 168 (Ch1) may function as a downstream seed and upstream LOthroughout system 100, while second unmodulated signal 170 (Ch1′)concurrently may function as an upstream seed and downstream LO forsystem 100.

According to the exemplary embodiment, within optical hub 102, firstunmodulated signal 168 (Ch1) is divided by hub optical splitter 130 andis separately input to both downstream transmitter 126 and upstreamreceiver 132 as a “pure” signal, and i.e., substantially low amplitude,narrow bandwidth continuous wave does not include adhered data. Firstunmodulated signal 168 (Ch1) thus becomes a seed signal for downstreamtransmitter 126 and an LO signal for upstream receiver 132. In anexemplary embodiment, within downstream transmitter 126, firstunmodulated signal 168 (Ch1) passes through downstream opticalcirculator 136 into downstream modulator 138, in which one or more laserdiodes (not shown in FIG. 1, described below with respect to FIGS. 2-5)are excited, and adhere data (also not shown in FIG. 1, described belowwith respect to FIGS. 2-5) to the signal that then exits downstreamoptical circulator 136 as downstream modulated data stream 172 (Ch1).

In an exemplary embodiment, downstream optical circulator 136 is withindownstream transmitter 126. Alternatively, downstream optical circulator136 may be physically located separately from downstream transmitter126, or else within the confines of downstream modulator 138. Downstreammodulated data stream 172 (Ch1) is then combined in hub opticalmultiplexer 128 with the plurality of modulated/unmodulated data streampairs from other channels (not shown) and transmitted over downstreamfiber 108, to a node optical demultiplexer 174 in fiber node 104, whichthen separates the different channel stream pairs for transmission todifferent respective end users 106. At end user 106, because the datastream pair 170, 172 entering downstream receiver 150 is phasesynchronized, digital signal processing at downstream DSP 156 is greatlysimplified, as described below with respect to FIG. 7.

Where upstream reception is optionally sought at optical hub 102, secondunmodulated signal 170 (Ch1′) is divided, within end user 106, by enduser optical splitter 158 and is separately input to both downstreamreceiver 150 and upstream transmitter 160 as a “pure” unmodulated signalfor Ch1′. In this alternative embodiment, second unmodulated signal 170(Ch1′) thus functions a seed signal for upstream transmitter 160 and a“pseudo LO signal” for downstream receiver 150 for the coherentdetection of Ch1. For purposes of this discussion, second unmodulatedsignal 170 (Ch1′) is referred to as a “pseudo LO signal” because it usesan LO signal from a remote source (output from first hub opticaldemultiplexer 124), and is not required to produce an LO signal locallyat end user 106. This particular configuration further significantlyreduces cost and complexity of the architecture of the system 100 by thereduction of necessary electronic components.

For upstream transmission, in an exemplary embodiment, a similarcoherent detection scheme is implemented for upstream transmitter 160 asis utilized for downstream transmitter 126. That is, second unmodulatedsignal 170 (Ch1′) is input to upstream optical circulator 162 andmodulated by upstream modulator 164 to adhere symmetric or asymmetricdata (not shown, described below with respect to FIG. 6) utilizing oneor more slave lasers (also not shown, described below with respect toFIG. 6), and then output as an upstream modulated data stream 176(Ch1′), which is then combined with similar modulated data streams fromother channels (not shown) by a node multiplexer 178 in fiber node 104.Second unmodulated signal 170 (Ch1′) is then transmitted upstream overupstream fiber 110, separated from other channel signals by second huboptical demultiplexer 134, an input to upstream receiver 132, forsimplified digital signal processing similar to the process describedabove with respect to downstream receiver 150.

By this exemplary configuration, multiple upstream channels fromdifferent end users 106 can be multiplexed at fiber node 104 (or aremote node) and sent back to optical hub 102. Thus, within optical hub102, the same coherent detection scheme may be used at upstream receiver132 as is used with downstream receiver 150, except that upstreamreceiver 132 utilizes first unmodulated signal 168 (Ch1) as the LO andupstream modulated data stream 176 (Ch1′) to carry data, whereasdownstream receiver 150 utilizes the data stream pair (Ch1, Ch1′) inreverse. That is, downstream receiver 150 utilizes second unmodulatedsignal 170 (Ch1′) as the LO and downstream modulated data stream 172(Ch1) to carry data.

Implementation of the embodiments described herein are useful formigrating hybrid fiber-coaxial (HFC) architectures towards other typesof fiber architectures, as well as deeper fiber architectures. TypicalHFC architectures tend to have very few fiber strands available fromfiber node to hub (e.g. fibers 108, 110), but many fiber strands couldbe deployed to cover the shorter distances that are typical from legacyHFC nodes to end users (e.g., fiber optics 112). In the exemplaryembodiments described herein, two fibers (i.e., fibers 108, 110) areillustrated between optical hub 102 and fiber node 104, which can be alegacy HFC fiber node. That is, one fiber (i.e., downstream fiber 108)is utilized for downstream signal and upstream seed/downstream LO, andanother fiber (i.e., upstream fiber 110) is utilized for upstreamsignal. Additionally, three fibers (i.e., fiber optics 112A-C) areillustrated for each end user from fiber node 104 (e.g., legacy HFCfiber node) to end user 106. By utilization of the advantageousconfigurations herein, fiber deeper or all-fiber migration schemes canutilize an HFC fiber node as an optical fiber distribution node, therebygreatly minimizing the need for fiber retrenching from an HFC node to anoptical hub.

The architecture described herein, by avoiding the need for conventionalcompensation hardware, can therefore be structured as a significantlyless expensive and more compact physical device than conventionaldevices. This novel and advantageous system and subsystem arrangementallows for multi-wavelength emission with simplicity, reliability, andlow cost. Implementation of optical frequency comb generator 114, withhigh quality input source signal 116, further allows simultaneouscontrol of multiple sources that are not realized by conventionaldiscrete lasers. According to the embodiments herein, channel spacing,for example, may be 25 GHz, 12.5 GHz, or 6.25 GHz, based on availablesignal bandwidth occupancy.

The embodiments described herein realize still further advantages byutilizing a comb generator (i.e., optical frequency comb generator 114)that maintains a constant wavelength spacing, thereby avoiding opticalbeat interference (OBI) that may be prevalent in cases with simultaneoustransmissions over a single fiber. In the exemplary embodimentillustrated in FIG. 1, fiber node 104 is shown as a passive system, andis thus expected to maintain a higher reliability than other migrationapproaches. Nevertheless, one of ordinary skill in the art, afterreading and comprehending present application, will understand how theembodiments disclosed herein may also be adapted to a remote PHYsolution, or to a remote CMTS that is included in the fiber node.

As illustrated and described herein, system 100 may utilize anarchitecture of coherent DWDM-PON incorporate novel solutions to meetthe unique requirements of access environment, but with cost-efficientstructures not seen in conventional hardware systems. Optical frequencycomb generator 114 produces a plurality of simultaneous narrow widthwavelength channels with controlled spacing, thereby allowing simplifiedtuning of the entire wavelength comb. This centralized comb light sourcein optical hub 102 therefore provides master seeding sources and LOsignals for both downstream and upstream directions in heterodynedetection configurations in order to reuse the optical sourcesthroughout the entirety of system 100. This advantageous configurationrealizes significant cost savings and reduction in hardware complexityover intradyne detection schemes in long-haul systems, for example.

FIG. 2 is a schematic illustration depicting an exemplary downstreamtransmitter 200 that can be utilized with fiber communication system100, depicted in FIG. 1. Downstream transmitter 200 includes downstreamoptical circulator 136 (see FIG. 1, above) in two-way communication witha laser injected modulator 202, which includes a laser diode 204, whichreceives data 206 from an external data source 208. In an alternativeembodiment, downstream transmitter 200 may include two separate fiberreceivers (not shown), which would substitute, and eliminate the need,for downstream optical circulator 136 in the structural configurationshown.

In operation, downstream transmitter 200 performs the same generalfunctions as downstream transmitter 126 (FIG. 1, described above). Laserinjected modulator 202 utilizes laser diode 204 as a “slave laser.” Thatis, laser diode 204 is injection locked by external laser 118, whichfunctions as a single frequency or longitudinal mode master, or seed,laser to keep the frequency of a resonator mode of laser diode 204 closeenough to the frequency of the master laser (i.e., laser 118) to allowfor frequency locking. The principle of downstream transmitter 200 isalso referred to as “laser cloning,” where a single high quality masterlaser (i.e., laser 118) transmits a narrow bandwidth, low noise signal(i.e., source signal 116), and a relatively inexpensive slave laser(e.g., laser diode 204) can be used throughout system 100 to transmitdata modulated signals, such as downstream modulated data stream 172(Ch1). In an exemplary embodiment, laser diode 204 is a Fabry Perotlaser diode (FP LD), or a vertical-cavity surface-emitting laser(VCSEL), in comparison with the considerably more expensive distributedfeedback laser diodes (DFB LD) that are conventionally used. In analternative embodiment, laser diode 204 is an LED, which can perform asa sufficient slave laser source according to the embodiments herein dueto the utilization of the high quality source signal 116 that isconsistently utilized throughout system 100.

More specifically, first unmodulated signal 168 (Ch1) exiting huboptical splitter 130 is input to downstream optical circulator 136,which then excites laser diode 204, that is, laser diode 204 emits lightat a specified modulation rate. Laser injected modulator 202 adheresdata 206 to the excited Ch1 signal, and the resultant modulated Ch1signal with adhered data is output from downstream optical circulator136 as downstream modulated data stream 172 (Ch1). According to thisexemplary embodiment, first unmodulated signal 168 (Ch1) is input todownstream transmitter 126 as an unmodulated, low amplitude, narrowbandwidth, low noise “pure” source, and is modulated by laser diode 204,which is a high amplitude, wide bandwidth device, and resultantdownstream modulated data stream 172 (Ch1) is a high amplitude, narrowbandwidth, low noise “pure” signal that can be transmitted throughoutsystem 100 without the need for further conventional compensation means(hardware and programming). Suppression of adjacent longitudinal modesfrom laser diode 204, for example, is not necessary because of theexciting source signal (i.e., signal 168) is of such high quality andnarrow bandwidth that output downstream modulated data stream 172 (Ch1)is substantially amplified only within the narrow bandwidth of externallaser 118. In the exemplary embodiment illustrated in FIG. 2, laserinjected modulator 202 implements direct modulation.

Optical injection locking as described herein thus improves upon theperformance of the relatively less expensive, multi-longitudinal slavelaser source (i.e., laser diode 204) in terms of spectral bandwidth andnoise properties. With respect to heterodyne coherent detection,incoming signals (upstream or downstream) can be combined with the LO orpseudo-LO and brought to an intermediate frequency (IF) for electronicprocessing. According to this exemplary configuration, part of theLO/pseudo-LO optical power can also be employed as the master/seed laserfor the reverse transmission direction, at both optical hub 102, and atend user 106 (described below with respect to FIG. 6), and thus a fullycoherent system having a master seed and LO delivery from an optical hubcan be achieved in a relatively cost-effective manner comparison withconventional systems.

FIG. 3 is a schematic illustration depicting an alternative downstreamtransmitter 300 that can be utilized with fiber communication system100, depicted in FIG. 1. Downstream transmitter 300 is similar todownstream transmitter 200 (FIG. 2), including the implementation ofdirect modulation, except that downstream transmitter 300 alternativelyutilizes polarization division multiplexing to modulate the Ch1 signalinto downstream modulated data stream 172 (Ch1).

Downstream transmitter 300 includes downstream optical circulator 136(see FIG. 1, above) in two-way communication with a laser injectedmodulator 302, which includes a polarization beam splitter(PBS)/polarization beam combiner (PBC) 304, which can be a singledevice. Laser injected modulator 302 further includes a first laserdiode 306 configured to receive first data 308 from an external datasource (not shown in FIG. 3), and a second laser diode 310 configured toreceive second data 312 from the same, or different, external datasource.

In operation, downstream transmitter 300 is similar to downstreamtransmitter 200 with respect to the implementation of direct modulation,and master/slave laser injection locking. Downstream transmitter 300though, alternatively implements dual-polarization from the splitterportion of PBS/PBC 304, which splits first unmodulated signal 168 (Ch1)into its x-polarization component P1 and y-polarization component P2,which separately excite first laser diode 306 and second laser diode310, respectively. Similar to downstream transmitter 200 (FIG. 2), indownstream transmitter 300, first unmodulated signal 168 (Ch1) exitinghub optical splitter 130 is input to downstream optical circulator 136,the separate polarization components of which then excite laser diodes306, 310, respectively, at the specified modulation rate. Laser injectedmodulator 302 adheres data first and second data 308, 312 to therespective excited polarization components of the Ch1 signal, which arecombined by the combiner portion of PBS/PBC 304. The resultant modulatedCh1 signal with adhered data is output from downstream opticalcirculator 136 as downstream modulated data stream 172 (Ch1).

In an exemplary embodiment, the polarized light components received byfirst and second laser diodes 306, 310 are orthogonal (90 degrees and/ornoninteractive). That is, first laser diode 306 and second laser diode310 are optimized as slave lasers to lock onto the same wavelength asexternal laser 118 (master), but with perpendicular polarizationdirections. By this configuration, large data packets (e.g., first data308 and second data 312) can be split and simultaneously sent alongseparate pathways before recombination as downstream modulated datastream 172 (Ch1). Alternatively, first data 308 and second data 312 maycome from two (or more) separate unrelated sources. The orthogonal splitprevents data interference between the polarized signal components.However, one of ordinary skill in the art will appreciate that,according to the embodiment of FIG. 3, first unmodulated signal 168(Ch1) can also be polarized at 60 degrees, utilizing similar principlesof amplitude and phase, as well as wavelength division. Firstunmodulated signal 168 (Ch1) can alternatively be multiplexed accordingto a spiral or vortex polarization, or orbital angular momentum.Additionally, whereas the illustrated embodiment features polarizationmultiplexing, space division multiplexing and mode division multiplexingmay be also alternatively implemented.

According to this exemplary embodiment, master continuous wave signalfor Ch1, namely, first unmodulated signal 168, is received from opticalfrequency comb generator 114 and is split to be used, in the first part,as the LO for upstream receiver 132, and in the second part, tosynchronize two slave lasers (i.e., first laser diode 306 and secondlaser diode 310) by the respective x-polarization and y-polarizationlight portions such that both slave lasers oscillate according to thewavelength of the master laser (i.e., external laser 118). Data (i.e.,first data 308 and second data 312) is directly modulated onto the twoslave lasers, respectively. This injection locking technique thusfurther allows for frequency modulation (FM) noise spectrum control fromthe master laser to the slave laser, and is further able to realizesignificant improvements in FM noise/phase jitter suppression andemission linewidth reduction.

As described herein, utilization of optical injection with adual-polarization optical transmitter (i.e., downstream transmitter 300)by direct modulation may advantageously implement relatively lower-costlasers to perform the functions of conventional lasers that areconsiderably more costly. According to this configuration of adual-polarization optical transmitter by direct modulation ofsemiconductor laser together with coherent detection, the presentembodiments are particular useful for short-reach applications in termsof its lower cost and architectural compactness. Similar advantages maybe realized for long reach applications.

FIG. 4 is a schematic illustration depicting an alternative downstreamtransmitter 400 that can be utilized with fiber communication system100, depicted in FIG. 1. Downstream transmitter 400 is similar todownstream transmitter 200 (FIG. 2), except that downstream transmitter400 alternatively implements external modulation, as opposed to directmodulation, to modulate the Ch1 signal into downstream modulated datastream 172 (Ch1). Downstream transmitter 400 includes downstream opticalcirculator 136 (see FIG. 1, above) and a laser injected modulator 402.Downstream optical circulator 136 is in one-way direct communicationwith a separate external optical circulator 404 that may be containedwithin laser injected modulator 402 or separate. Laser injectedmodulator 402 further includes a laser diode 406, which receives the lowamplitude, narrow bandwidth, first unmodulated signal 168 (Ch1) andemits an excited, high amplitude, narrow bandwidth, optical signal 408back to external optical circulator 404. Laser injected modulator 402still further includes an external modulating element 410, whichreceives data 412 from an external data source 414, and adheres data 412with optical signal 408 to be unidirectionally received back bydownstream optical circulator 136 and output as downstream modulateddata stream 172 (Ch1).

In this exemplary embodiment, downstream transmitter 400 performs thesame general functions as downstream transmitter 126 (FIG. 1, describedabove), but uses external modulation as the injection locking mechanismto lock laser diode 406 to the wavelength of the master laser source(e.g., external laser 118). To implement external modulation, thisembodiment regulates optical signal flow through mostly unidirectionaloptical circulators (i.e., downstream optical circulator 136, externaloptical circulator 404). External modulating element 410 may optionallyinclude a demultiplexing filter (not shown) as an integral component, orseparately along the signal path of downstream modulated data stream 172(Ch1) prior to input by downstream receiver 150. In an exemplaryembodiment, external modulating element 410 is a monitor photodiode, andinjection locking is performed through a rear laser facet.

FIG. 5 is a schematic illustration depicting an alternative downstream500 transmitter that can be utilized with fiber communication system100, depicted in FIG. 1. Downstream transmitter 500 is similar todownstream transmitter 300 (FIG. 3), including the implementation ofdirect modulation and polarization division multiplexing, except thatdownstream transmitter 500 further implements quadrature amplitudemodulation (QAM) to modulate the Ch1 signal into downstream modulateddata stream 172 (Ch1). That is, further external modulating elements maybe utilized per polarization branch (FIG. 2, above) to generate QAMsignals.

Downstream transmitter 500 includes downstream optical circulator 136(see FIG. 1, above) in two-way communication with a laser injectedmodulator 502, which includes a PBS/PBC 504, which can be a singledevice or two separate devices. Additionally, all of the components oflaser injected modulator 502 may themselves be separate devices, oralternatively all contained within a single photonic chip. Laserinjected modulator 502 further includes a first laser diode 506configured to receive first data 508 from an external data source (notshown in FIG. 5), a second laser diode 510 configured to receive seconddata 512 from the same, or different, external data source, a thirdlaser diode 514 configured to receive third data 516 from thesame/different, external data source, and a fourth laser diode 518configured to receive fourth data 520 from the same/different externaldata source.

In operation, downstream transmitter 500 implements dual-polarizationfrom the splitter portion of PBS/PBC 504, which splits first unmodulatedsignal 168 (Ch1) into its x-polarization component (P1) andy-polarization component (P2). Each polarization component P1, P2 isthen input to first non-polarized optical splitter/combiner 522 andsecond non-polarized optical splitter/combiner 524, respectively. Firstand second optical splitters/combiners 522, 524 each then further splittheir respective polarization components P1, P2 into their I-signals526, 528, respectively, and also into their Q-signals 530, 532,respectively. Generated I-signals 526, 528 then directly excite laserdiodes 506, 514, respectively. Before directly communicating with laserdiodes 510, 518, respectively, generated Q-signals 530, 532 first passthrough first and second quadrature phase shift elements 534, 536,respectively, each of which shifts the Q-signal by 45 degrees in eachdirection, such that the respective Q-signal is offset by 90 degreesfrom its respective I-signal when recombined at splitters/combiners 522,524.

The resultant modulated Ch1 signal, with adhered data, is output fromdownstream optical circulator 136 of downstream transmitter 500 asdownstream modulated data stream 172 (Ch1), and as a polarized,multiplexed QAM signal. According to this exemplary embodiment,utilization of a photonic integrated circuit allows for directlymodulated polarization of a multiplexed coherent system, but atsignificantly lower cost hardware configurations than could be realizedby conventional architectures. In an exemplary embodiment, laser diodes506, 510, 514, 516 are PAM-4 modulated laser diodes capable ofgenerating 16-QAM polarization multiplexed signals.

FIG. 6 is a schematic illustration depicting an exemplary upstreamtransmitter 600 that can be utilized with the fiber communication system100, depicted in FIG. 1. In the embodiment illustrated in FIG. 6,upstream transmitter 600 is similar to downstream transmitter 300 (FIG.3) in structure and function. Specifically, upstream transmitter 600includes upstream optical circulator 162 (see FIG. 1, above) in two-waycommunication with a laser injected modulator 602 (not separatelyillustrated in FIG. 6), which includes a PBS/PBC 604, which can be asingle device or separate devices. Laser injected modulator 602 furtherincludes a first laser diode 606 configured to receive first data 608from an external data source (not shown in FIG. 6), and a second laserdiode 610 configured to receive second data 612 from the same, ordifferent, external data source. Similar to the embodiments of FIGS.2-5, above, downstream transmitter 600 may also eliminate for upstreamoptical circulator 162 by the utilization of at least two separate fiberreceivers (not shown).

Upstream transmitter 600 is thus nearly identical to downstreamtransmitter 300 (FIG. 3), except that upstream transmitter 600 utilizessecond unmodulated signal 170 (Ch1′) as the end user seed source, inlaser injected modulator 602, to combine or adhere with data (e.g.,first data 608, second data 612) to generate upstream modulated datastream 176 (Ch1′) to carry upstream data signals to an upstream receiver(e.g., upstream receiver 132). In operation, first laser diode 606 andsecond laser diode 610 also function as slave lasers by injectionlocking to the master signal from external laser 118. That is, symmetricor asymmetric data for Ch1′ (e.g., first data 608, second data 612) ismodulated onto the two slave lasers (i.e., first laser diode 606 andsecond laser diode 610) with polarization multiplexing, much the same asthe process implemented with respect to downstream transmitter 300 (FIG.3) in optical hub 102.

In this example, upstream transmitter 600 is illustrated tosubstantially mimic the architecture of downstream transmitter 300 (FIG.3). Alternatively, upstream transmitter 600 could equivalently mimic thearchitecture of one or more of downstream transmitters 200 (FIG. 2), 400(FIG. 4), or 500 (FIG. 5) without departing from the scope of thepresent disclosure. Furthermore, upstream transmitter 600 can conform toany of the embodiments disclosed by FIGS. 2-5, irrespective of thespecific architecture of the particular downstream transmitter utilizedwithin optical hub 102. By utilization of high-quality, narrowbandwidth, low noise external laser source 118, the master/slave laserrelationship carries through the entirety of system 100, and theplurality of end users 106 that receive modulated/unmodulated signalpairs (which may be 32, 64, 128, or as many as 256 from a single fiberline pair, e.g., downstream fiber 108 and upstream fiber 110).

The significant cost savings according to the present embodiments arethus best realized when considering that as many as 512 downstreamtransmitters (e.g., downstream transmitter 126, FIG. 1) and upstreamtransmitters (e.g., upstream transmitter 160, FIG. 1) may be necessaryto fully implement all available chattel pairs from a single optical hub102. The present embodiments implement a significantly lower cost andless complex hardware architecture to utilize the benefits accruing fromimplementation of high-quality external laser 118, without having to addexpensive single longitudinal mode laser diodes, or other compensationhardware necessary to suppress adjacent longitudinal modes frominexpensive lasers or the noise components produced thereby.

FIG. 7 is a schematic illustration depicting an exemplary processingarchitecture which can be implemented for upstream receiver 132,downstream receiver 150, and fiber communication system 100, depicted inFIG. 1. The respective architectures of upstream receiver 132 anddownstream receiver 150 are similar with respect to form and function(described above with respect to FIG. 1), except that upstream receiver132 receives a first data stream pair 700 for Ch1, Ch1′, in reverse of asecond data stream pair 702, which is received by downstream receiver150. In other words, as described above, first data stream pair 700includes first unmodulated signal 168 (Ch1) as the LO and upstreammodulated data stream 176 (Ch1′) to carry data, whereas second datastream pair 702 includes unmodulated signal 170 (Ch1′) as the LO anddownstream modulated data stream 172 (Ch1) to carry data.

First and second data stream pairs 700, 702 the multiplexed phasesynchronized pairs modulated/unmodulated of optical signals that areconverted into analog electrical signals by ICR 140 and ICR 152,respectively. The respective analog signals are then converted intodigital domain by ADC 142 and ADC 154, for digital signal processing byDSP 144 and DSP 156. In an exemplary embodiment, digital signalprocessing may be performed by a CMOS ASIC employing very largequantities of gate arrays. A conventional CMOS ASIC, for example, canutilize as many as 70 million gates to process incoming digitized datastreams. In the conventional systems, modulated data streams for Ch1 andCh1′ are processed independently, which requires significant resourcesto estimate frequency offset, drift, and digital down conversioncompensation factors (e.g., e{circumflex over ( )}-jωt, where ωrepresents the frequency difference between first unmodulated signal 168and upstream modulated data stream 176, and w is held constant forcoherent tone pair 166, as extended throughout system 100).

According to the exemplary embodiments disclosed herein, on the otherhand, the modulated and unmodulated signals from Ch1 and Ch1′ are phasesynchronized together such that the difference between w of the signalpair is always known, and phase synchronized to maintain a constantrelationship. In contrast, conventional systems are required toconstantly estimate the carrier phase to compensate for factors such asdraft which requires considerable processing resources, as discussedabove. According to the present embodiments though, since Ch1 and Ch1′are synchronized together as first and second data stream pairs 700,702, the offset ω between the pairs 700, 702 need not be estimated,since it may be instead easily derived by a simplified subtractionprocess in DSP 144 and DSP 156 because the signal pairs will drifttogether by the same amount in a constant relationship. By thisadvantageous configuration and process, digital signal processing by aCMOS ASIC can be performed utilizing as few as one million gates,thereby greatly improving the processing speed of the respective DSP,and/or reducing the number of physical chips required to perform theprocessing (or similarly increasing the amount of separate processingthat may be performed by the same chip). At present, implementation ofthe embodiments described herein may improve downstream and upstreamdata transmission speeds by as much as 5000 times faster thanconventional systems.

FIG. 8 is a flow chart diagram of an exemplary downstream opticalnetwork process 800 that can be implemented with fiber communicationsystem 100, depicted in FIG. 1. Process 800 begins at step 802. In step802, coherent tone pairs 166 are generated and output by opticalfrequency comb generator 114, amplifier 122, and first hub opticaldemultiplexer 124. Similar to the discussion above, for simplificationpurposes, the following discussion addresses specific coherent tone pair166(1) for Ch1, Ch1′. Coherent tone pair 166 includes first unmodulatedsignal 168 (Ch1) and second unmodulated signal 170 (Ch1′). Once coherenttone pair 166 is generated, process 800 proceeds from step 802 to steps804 and 806, which may be performed together or simultaneously.

In step 804, first unmodulated signal 168 (Ch1) is input to an opticalsplitter, e.g., optical splitter 130, FIG. 1. In step 806, secondunmodulated signal 170 (Ch1′) is transmitted to a multiplexer, e.g., huboptical multiplexer 128, FIG. 1. Referring back to step 804, firstunmodulated signal 168 (Ch1) is split to function both as an LO forupstream detection, and as a seed for downstream data transmission. Forupstream detection, step 804 proceeds to step 808, where firstunmodulated signal 168 (Ch1) is received by an upstream receiver, i.e.,upstream receiver 132, FIG. 1. For downstream data transmission, step804 separately and simultaneously proceeds to step 810.

Step 810 is an optional step, where polarization division multiplexingis desired. In step 810, first unmodulated signal 168 (Ch1) is splitinto its x-component and y-component parts P1, P2, respectively (e.g.,by PBS/PBC 304, FIG. 3 or PBS/PBC 504, FIG. 5) for separate direct orexternal modulation. Where polarization division multiplexing is notutilized, process 800 skips step 810, and instead proceeds directly fromstep 804 to step 812. In step 812, first unmodulated signal 168 (Ch1),or its polarized components if optional step 810 is implemented, ismodulated by direct (e.g., FIGS. 2, 3, 5) or external (e.g., FIG. 4)modulation. Process 800 then proceeds from step 812 to step 814. Step814 is an optional step, which is implemented if optional step 810 isalso implemented for polarization division multiplexing. In step 814,the x-component and y-component parts P1, P2 are recombined (e.g., byPBS/PBC 304, FIG. 3 or PBS/PBC 504, FIG. 5) for output as downstreammodulated data stream 172 (Ch1). Where polarization divisionmultiplexing was not utilized, process 800 skips step 814, and insteadproceeds directly from step 812 to step 816.

In step 816, second unmodulated signal 170 (Ch1′) and downstreammodulated data stream 172 (Ch1) are optically multiplexed, i.e., by huboptical multiplexer 128, FIG. 1, as a phase synchronized data streampair (e.g., second data stream pair 702, FIG. 7). Process 800 thenproceeds from step 816 to step 818, where the phase synchronized datastream pair is transmitted over an optical fiber, i.e., downstream fiber108, FIG. 1. Process 800 then proceeds from step 818 to step 820, wherethe synchronized data stream pair is optically demultiplexed, e.g., bynode optical demultiplexer 174 in fiber node 104. Process 800 thenproceeds from step 820 to step 822, where both components of thedemultiplexed data stream pair (e.g., second unmodulated signal 170(Ch1′) and downstream modulated data stream 172 (Ch1)) are received by adownstream receiver (e.g., downstream receiver 150, FIG. 1) forheterodyne coherent detection.

Where an end user (e.g., end user 106) further includes upstreamtransmission capability, process 800 further includes optional steps 824and 826. In step 824, and prior to downstream reception in step 822,second unmodulated signal 170 (Ch1′) is optically split (e.g., by enduser optical splitter 158, FIG. 1), and additionally transmitted, instep 826, to an upstream transmitter of the end user (e.g., upstreamtransmitter 160, FIG. 1) as a seed signal for a modulator (e.g.,modulator 164, FIG. 1) for upstream data transmission, as explainedfurther below with respect to FIG. 9.

FIG. 9 is a flow chart diagram of an exemplary upstream optical networkprocess 900 that can be optionally implemented with fiber communicationsystem 100, depicted in FIG. 1. Process 900 begins at optional step 902.In step 902, where polarization division multiplexing is utilized in theupstream transmitter (e.g., upstream transmitter 160, FIG. 1), secondunmodulated signal 170 (Ch1′) (from step 826, FIG. 8) is split into itsx-component and y-component parts (e.g., by PBS/PBC 604, FIG. 6) forseparate direct or external modulation. Where polarization divisionmultiplexing is not utilized, step 902 is skipped, and process 900instead begins at step 904.

In step 904, second unmodulated signal 170 (Ch1′), or its polarizedcomponents if optional step 902 is implemented, is injection locked tothe master source laser (e.g., external laser 118, FIG. 1), as describedabove with respect to FIGS. 1 and 6. Step 904 then proceeds to step 906,where injection locked signal is modulated by direct or externalmodulation. Process 900 then proceeds from step 906 to step 908. Step908 is an optional step, which is implemented if optional step 902 isalso implemented for polarization division multiplexing. In step 908,the x-component and y-component parts of the excited Ch1′ signal arerecombined (e.g., by PBS/PBC 604, FIG. 6) for output as upstreammodulated data stream 176 (Ch1′). Where polarization divisionmultiplexing was not utilized, process 900 skips step 908, and insteadproceeds directly from step 906 to step 910.

In step 910, upstream modulated data stream 176 (Ch1′) is opticallymultiplexed, i.e., by node optical multiplexer 178, FIG. 1, with otherupstream data stream signals (not shown). Process 900 then proceeds fromstep 910 to step 912, where upstream modulated data stream 176 (Ch1′) istransmitted over an optical fiber, i.e., upstream fiber 110, FIG. 1.Process 900 then proceeds from step 912 to step 914, where upstreammodulated data stream 176 (Ch1′) is optically demultiplexed, e.g., bysecond hub optical demultiplexer 134, which separates the selected datastream from the other upstream data stream signals, for transmission toa particular upstream receiver tuned to receive the modulated datastream. Process 900 then proceeds from step 914 to step 916, where bothcomponents (e.g., first unmodulated signal 168 (Ch1), FIG. 8, andupstream modulated data stream 176 (Ch1′)) of the upstream data streampair, e.g., first data stream pair 700, FIG. 7, are received by anupstream receiver (e.g., upstream receiver and 32, FIG. 1) forheterodyne coherent detection.

As illustrated in the exemplary embodiment, a difference betweenupstream and downstream signal transmission is that an entiresynchronized modulated/unmodulated channel pair (e.g., second datastream pair 702, FIG. 7) can be transmitted in the downstream direction,whereas, in the upstream direction, only a data modulated signal (e.g.,upstream modulated data stream 176 (Ch1′)) to be transmitted over theupstream fiber connection, i.e., upstream fiber 110. An advantage of thepresent configuration is that the LO for upstream coherent detection(e.g., at upstream receiver 132, FIG. 1) comes directly from the splitsignal, i.e., first unmodulated signal 168 (Ch1) generated from opticalfrequency comb generator 114 within optical hub 102, after separation byfirst hub optical demultiplexer 124, as depicted in FIG. 1. Conventionalsystems typically require LO generation at each stage of the respectivesystem. According to the present disclosure, on the other hand,relatively inexpensive slave lasers can be implemented throughout thesystem architecture for modulation and polarization multiplexing in bothoptical hub 102 and end user 106 components, without requiring anadditional LO source at the end user.

According to the present disclosure, utilization of dual-polarizationoptical transmitters, and by direct modulation of semiconductor laserswith coherent detection, is particularly beneficial for not onlylonghaul applications, but also for shortreach applications to reducethe cost of electronic hardware, while also rendering the overallnetwork system architecture more compact. The present systems andmethods further solve the conventional problem of synchronizing twolaser sources over a long period of time. Utilization of the phasesynchronized data stream pairs and slave lasers herein allows continualsynchronization of the various laser sources throughout the systemduring its entire operation. These solutions can be implemented withincoherent DWDM-PON system architectures for access networks in acost-efficient manner.

Utilization of the high quality optical comb source at the front end ofthe system thus further allows a plurality of simultaneous narrowbandwidth wavelength channels to be generated with easily controlledspacing, and therefore also simplified tuning of the entire wavelengthcomb. This centralized comb light source in the optical hub providesmaster seeding sources and LO signals that can be reused throughout thesystem, and for both downstream and upstream transmission. Theimplementation of optical injection, as described herein, furtherimproves the performance of low-cost multi-longitudinal slave lasersources in terms of spectral bandwidth and noise properties. Accessnetworks according to the present systems and methods thus achieve moreefficient transmission of wavelengths through optical fibers, therebyincreasing the capacity of transmitted data, but at lower power,increased sensitivity, lower hardware cost, and a reduction indispersion, DSP compensation, and error correction.

Injection Locking Systems and Methods

As described above, the innovative access network architectures of thepresent embodiments that implement coherent optics technology form thefoundation of achieving transport speeds of 100G and beyond in theaccess network paradigm. The coherent modulation schemes described aboveadvantageously enable an access network to drive down cost andcomplexity due to the spectral efficiency of coherent optics technology,as well as the management simplicity of the present systems and methods.

In a typical access network, much of the cost per bit of the network isfixed (e.g., by the fiber, line systems, facilities, management system,etc.), and largely independent of the data rate. Thus, the cost per bitin the access network will be most significantly lowered by achieving ahigher data rate. In a practical implementation of the embodimentsdescribed above, the present inventors have already achieved asingle-direction capacity of over 8,000 Gb/s on a single fiber, whichrepresents approximately 5,000 times the capacity of access networksemploying analog optics technology. This capacity is projected toincrease to approximately 25,000 Gb/s per fiber within the next year,50,000 Gb/s within the following few years, and greater within the nextdecade. However, the bulk of the capacity improvements has beenprimarily directed toward P2P configurations; conventional directdetection techniques have not been designed to reach residential endusers in their homes.

The following systems and methods describe further improvements to thenovel coherent PON (CPON) embodiments, described above, that implementinjection locking technology from a master laser source. In exemplaryembodiments, the present injection locking techniques may be implementedwith respect to a master laser source generating a plurality of distinctcoherent CW signals, or with respect to a master laser source generatinga plurality of phase synchronized coherent tone pairs for eachwavelength (e.g., phased synchronized coherent tone source pairs 166,FIG. 1).

In an exemplary embodiment, a novel CPON architecture implements opticalinjection locking technology that results in a significant reduction tothe structural cost and operational complexity of an ONU. In someembodiments, homodyne detection techniques are implemented at the ONUutilizing downlink signals for both a master laser and a localoscillator. In other embodiments, heterodyne detection techniques areimplemented at the ONU using remote delivery for the master laser andthe local oscillator. According to these innovative the methods, thepresent CPON configurations advantageously overcome the significantlimitations of the conventional direct detection systems describedabove.

FIG. 10 is a schematic illustration of a P2MP CPON 1000. In an exemplaryembodiment, P2MP CPON 1000 includes a headend/hub 1002 in operablecommunicative coupling with a plurality of end users 1004 over atransport medium 1006. In this example, headend/hub 1002 may represent acentral office, and may additionally include at least one coherent OLTas the MTS thereof. In a similar manner, one or more of end users 1004may represent a residential home subscriber, and may include a separateONU as the modem thereof at each respective residential location.Additionally in this example, transport medium 1006 may be an opticalfiber having a length of 80 km, and configured to transport data at a120 Gb/s capacity.

In an embodiment, CPON 1000 may be a coherent optical network configuredto transmit a downstream coherent optical signal over transport medium1006, from headend/hub 1002 to a splitter 1008, which splits thedownstream coherent optical signal into a plurality of opticalwavelengths, for further transport, over a plurality of short fibers1010, to respective ONUs of end users 1004. In this example, the OLT ofheadend/hub 1002 may include at least one downstream coherent opticaltransmitter (not separately shown), and each ONU of an end-user 1004 mayinclude a counterpart downstream coherent optical receiver. In anembodiment, splitter 1008 may represent one or more of an opticalcombiner, an optical splitter, a wavelength multiplexer, a wavelengthdemultiplexer, an optical coupler, and/or combinations thereof.

In a practical implementation of P2MP CPON 1000, the present inventorshave successfully demonstrated a 120 Gb/s capacity, shared among 512subscriber end users 1004, per wavelength over 80 km of fiber 1006. Thepresent embodiments therefore realize significant improvements to thedownstream portion of the CPON solution, thereby providing a technicallyfeasible approach for both an extended reach, and also an ultra-highsplit ratio, for beyond the target 100G data rate capacity.

More particularly, the downstream solutions described herein have beensuccessfully proven to advantageously increase the downstream capacityreach and split ratio, and for both conventional communication networksas well as emerging full duplex coherent optical systems (i.e.,simultaneous bidirectional communication over the same fiber and samewavelength through coherent optics technology). The present systems andmethods further advantageously enable full utilization of a novelcoherent upstream burst transceiver, as well as all of the coherentoptics injection locking techniques described herein, both individuallyand in combination, while also providing for use of a significantlylow-cost laser at the customer premises of an end user. The novelcoherent upstream burst transceiver is the subject of differentapplication, whereas the following embodiments focus in greater detailon customer premises equipment, such as the ONU, which represents one ofthe most cost-sensitive portions of the network. As described furtherbelow, the systems and methods herein advantageously enable the use ofsignificantly less expensive lasers that simplify the coherenttransceiver configuration, which results in an ultra-low cost coherenttransceiver that is relatively affordable for implementation at eachsubscriber residential home location, in comparison with conventionaldirect detection communication systems.

FIG. 11 depicts a comparative distribution 1100 between the presentembodiments and conventional systems with respect to relative cost andcomplexity of structural components and algorithms. More specifically,although not specifically shown to scale, comparative distribution 1100illustrates the relative cost and complexity of the various DSP,receiver (RX), and laser hardware/software between a conventional P2PPON system 1102, and optimized access P2P PON system 1104, and a P2MPCPON system 1106 (e.g., P2MP CPON 1000, FIG. 10). In an exemplaryembodiment, comparative distribution 1100 represents the relative costand complexity improvements with respect to conventional long haul andmetro transceivers, in particular with respect to minimization of DSPcomplexity, and simplification and cost reduction of the receiver.

Nevertheless, reduction of the cost and complexity of the laser sourcehas been a significant challenge to conventional solutions. As describedabove, high-quality laser sources are known, but are too expensive to bepractically implemented at the location of each end user in a coherentP2MP network. The following embodiments thus demonstrate an innovativesolution for inexpensively providing a high-quality, less complex, lasersource at each downstream receiver/upstream transmitter of an end user(e.g., the ONU).

FIG. 12A depicts an exemplary use case of an ECL laser source 1200. Inan embodiment, ECL laser source 1200 includes an ECL laser 1202configured to generate an optical source output beam 1204 having anarrow linewidth, and exhibiting a distribution pattern 1206 havingrelatively low noise. In this example, ECL laser 1202 may be aconventional, high-quality ECL laser having a very narrow line-width andsignificantly low noise output, i.e., in comparison with knownlower-quality lasers having wide linewidths and more noise. The qualityof laser sources is an important factor in the implementation ofcoherent optical transmission systems. However, as described above,high-quality lasers, such as ECL laser 1202, are prohibitively expensivefor home-based solutions. At present, a conventional ECL laser costapproximately $1500 US.

FIG. 12B depicts an exemplary use case of a FP laser source 1208. Moreparticularly, in an embodiment, FP laser source 1208 includes an FPlaser 1210 configured to generate an optical source output beam 1212having a significantly wider linewidth and a distribution pattern 1214exhibiting significantly more noise (i.e., in comparison with outputbeam 1204 of ECL laser 1202, FIG. 12A). As described above, therelatively low-cost FP lasers are significantly more affordable and lesscomplex than the high-quality laser sources required for presentcoherent optics implementations. Prior to the prevalence of coherentoptics technology, FP lasers were commonly used in upstreamtransmissions of HFC deployments, but have not been applicable topresent-day coherent optical systems. According to the present systemsand methods though, these previously inapplicable low-cost laser sourcesare effectively transformed into significantly higher-performing lasers,but without significantly increasing the cost of such implementationsbeyond where the deployment is economically infeasible for home use.This solution is described further below with respect to FIG. 12C.

FIG. 12C depicts an exemplary hybrid system 1216 for injection lockingECL laser source 1200, FIG. 12A, into FP laser source 1208, FIG. 12B.More specifically, in an exemplary embodiment, hybrid system 1216employs a novel coherent optical injection locking (COIL) technique toeffectively “clone” the expensive, high quality, high performance of asingle laser source (e.g., ECL laser source 1200) onto a plurality ofrelatively very basic and simpler lasers (e.g., FP laser source 1208).An exemplary operation of the injection locking process, high-qualitylaser 1202 (relatively expensive) generates high-quality optical source1204, which is injected into FP laser 1210 through an optical circulator1218. Through this optical injection process, the opticalcharacteristics of the expensive ECL laser 1202 transfers to and outputoptical source 1220 of the considerably less expensive FP laser 1210.Output optical source 1220 this includes the frequency and linewidth ofoptical source 1204, which may then be readily utilized from anapparently “noisy” distribution pattern 1222, as important factors forthe coherent system utilizing this COIL technique.

According to the hybrid solution depicted in FIG. 12C, the cost of thecloned optical source is effectively reduced (i.e., in comparison withthe high-quality optical source being cloned) by approximately 200times. In the exemplary embodiment, high-quality laser 1202 remains atthe hub location (e.g., headend/hub 1002, FIG. 10), and this remotesignal may then be shared/transmitted to many customer premise locationswhere a cloned laser source is deployed (described further below withrespect to FIG. 13). Through this injection locking technique, eachlow-cost laser 1210 behaves like expensive laser 1202, but only oneexpensive laser 1202 is needed to implement this COIL technique for manylow-cost lasers 1210 in a single P2MP network.

Accordingly, utilizing this COIL technique, the cost of expensive laser1202 may be shared among and buying the many end users. In this example,in the case of 512 end users for a single expensive (i.e., approximately$1500 US) laser 1202, the expense of deploying an individual low-costlaser 1210 for each end user is approximately $40 US per residentialhome location implementing COIL. This cloning technique thereforerepresents a drastic cost saving solution over the deployment of anindividual high-quality laser 1202 at each home location (i.e., notutilizing this COIL technique). In this example, the total cost ofproviding a coherent laser source at each individual home locationincludes the individual cost of cloned FP laser 1210, together with athe respective shared portion of the costs of parent laser 1202 andoptical circulator 1218. The cost-per-home utilizing this COIL techniqueis thus in the approximate range of conventional direct detection PONsystems. Accordingly, through the present systems and methods, coherenttechnology that is presently available to P2P network business users,may be affordably provided to residential subscribers in a P2MP network.An exemplary merger of such P2P and P2MP services is illustrated furtherbelow with respect to FIG. 13.

FIG. 13 is a schematic illustration of an injection locked P2MP CPON1300. P2MP CPON 1300 is similar in several aspects to P2MP CPON 1000,FIG. 10, and similarly-labeled elements thereof may be generallyconsidered to have a substantially similar structure and or function.For example, similar to P2MP CPON 1000, P2MP CPON 1300 includes aheadend/hub 1302 in operable communication with a plurality of end users1304 over a transport medium/long fiber 1306, which connects to asplitter 1308 that splits the downstream coherent optical signal into aplurality of optical wavelengths for transport to respective ONUs of endusers 1304 over a plurality of short fibers 1310.

Different though, from P2MP CPON 1000, in an exemplary embodiment, P2MPCPON 1300 is further configured such that a plurality of P2P fibers 1312are communicatively coupled with headend/hub 1302 which includes atleast one parent laser source 1314 at an MTS (e.g., the OLT) thereof.Accordingly, each end user 1304 further includes at least one childlaser source 1316 at a modem (e.g., the ONU) thereof, and configured forinjection locking with parent laser source 1314 according to the COILtechniques described above. According to this exemplary P2MPconfiguration, 100G coherent optical technology services, which havebeen heretofore only financially within the reach of P2P businesssubscribers, may now be provided to residential homes at a greatlyreduced price that is competitive with conventional direct detection PONschemes.

As illustrated in FIG. 13, the cloning techniques of the presentinjection locking approach not only improves the P2MP paradigm, where asingle source (e.g., headend/hub 1302) is transmitted to multipledestinations (e.g., end users 1304), but also provides a uniquecapability to the reverse case, namely, multiple sources (e.g., P2Pfibers 1312) to a single destination (e.g., headend/hub 1302).Accordingly, in the near term, the injection locking technology of thepresent systems and methods it is further capable of reducing by atleast 20 percent the cost of a P2P coherent link to a node in theDistributed Access paradigm. Thus, the overall cost of coherent opticsdeployment is significantly reduced throughout the several accessparadigms, but without sacrificing delivery speed.

Further implementations of the present injection locking systems andmethods are described further below with respect to FIGS. 14-24, andmore specifically with respect to operation at a downlink coherentoptical transceiver. In the following embodiments, the downlink coherentoptical transceiver is described, for ease of explanation, to be an ONU.Nevertheless, the person of ordinary skill in the art will appreciatethat the innovative techniques of the present embodiments are applicableto other receivers, transceivers, and/or modems, and are furtherapplicable to uplink coherent optical transceiver configurations, asalso described above.

FIG. 14 is a schematic illustration of an exemplary PON system 1400implementing homodyne detection. In an exemplary embodiment, system 1400represents a CPON configured to implement COIL-based homodyne detection,and includes a downlink signal 1402 from a high quality parent/masterlaser source, which is transmitted as a downstream coherent wavelengthλ_(D) to an ONU 1404 over an optical fiber 1406. In an embodiment, ONU1404 includes a slave laser 1408 and one or more of a first splitter1410, and optical filter 1412, an optical circulator 1414, a secondsplitter 1416, a homodyne detection unit 1418, and an uplink modulator1420 configured to output a modulated wavelength λ_(U). In an exemplaryembodiment, system 1400 may be implemented at the end user location ofone or more of the respective systems described above, and downstreamcoherent wavelength λ_(D) serves as downlink signal 1402 for both themaster source and the LO.

In exemplary operation of system 1400, for the downlink, the receivedsignal λ_(D) is split by first splitter 1410 (e.g., a 3-dB opticalcoupler) with one arm thereof fed directly to homodyne detection unit1418 (i.e., the downlink coherent receiver), and the other arm filteredfirst through optical filter 1412 (e.g., a narrow optical filter) andthen injected, through optical circulator 1414 (e.g., a 3-port opticalcirculator), into the low-cost laser source of slave laser 1408. In theexemplary embodiment, optical filter 1412 is configured such that masterlaser power is sufficiently balanced with the narrow width of stripedsignal bandwidth.

After injection locking with slave laser 1408, the newly injected signalof a resultant injection locked carrier signal 1422 is split by secondsplitter 1416 (e.g., a 3-dB splitter), with one arm thereof returned tohomodyne detection unit/receiver 1418 as the LO with phase noise andcarrier frequency offset removed. According to this configuration,homodyne detection unit 1418 is advantageously capable of achievingcarrier recovery in the optical domain without any delay. The other armfrom second splitter 1416 is sent to uplink modulator 1422 become theuplink optical source of upstream modulated wavelength λ_(U), which maybe communicated upstream by way of a full duplex coherent opticalconnection schemes using an optical circulator (e.g., FIGS. 15-16,below), or realized by a dedicated separate, second fiber (e.g., FIG.17, below).

In an embodiment, in the case of asymmetrical modulation, the upstreamtransmission may employ an intensity modulation scheme, such as anon-return to zero on-off keying (NRZ-OOK) format, instead of thecoherent modulation scheme implemented for the downstream transmission.According to the advantageous configuration of system 1400, only onerelatively low-cost slave laser (e.g., an FP laser source) need beemployed at each ONU 1404 to realize the effective performance of thehigh-cost master laser (e.g., an ECL laser source). In the exemplaryembodiment, no carrier recovery is then needed in the relevant DSP floweither.

FIG. 15 is a schematic illustration of an exemplary PON system 1500implementing homodyne detection in a full duplex operation. In anexemplary embodiment, system 1500 represents a CPON configured toimplement COIL-based homodyne detection, in a substantially similarmanner to system 1400, FIG. 14, and includes a downlink signal 1502 _(D)from a high quality parent/master laser source, which is transmitted asa downstream coherent wavelength λ_(D) to an ONU 1504 over an opticalfiber 1506. In an embodiment, ONU 1504 includes a slave laser 1508 andone or more of a first splitter 1510, an optical filter 1512, an ONUoptical circulator 1514, a second splitter 1516, a homodyne detectionunit/receiver 1518, and an uplink modulator 1520 configured to output amodulated upstream wavelength λ_(U) from an injection locked carriersignal 1522. In an exemplary embodiment, system 1500 may be implementedat the end user location one or more of the respective systems describedabove, and downstream coherent wavelength λ_(D) again may serve asdownlink signal 1502 _(D) for both the master source and the LO.

In exemplary operation, system 1500 performs similarly to system 1400with respect to the reception and processing of downlink signal 1502_(D) by ONU 1504. Different though, from the embodiment depicted in FIG.14, system 1500 further includes a hub optical circulator 1524 disposedat the end of fiber 1506 that couples with the headend/hub (not shown inFIG. 15), and an end user optical circulator 1526 disposed at the otherend of fiber 1506 that couples with ONU 1504.

In an exemplary embodiment, both of hub optical circulator 1524 and enduser optical circulator 1526 are 3-port optical circulators respectivelydisposed on either side of the uplink and the downlink. In furtherexemplary operation, at the hub location, downlink signal 1502 _(D) byis transmitted from port 1 to port 2 of hub optical circulator 1524, andan uplink signal 1502 _(U) is transmitted from port 2 to port 3 of huboptical circulator 1524. Similarly, at the end point location of ONU1504, downlink signal 1502 _(D) by is transmitted from port 2 to port 3of end point optical circulator 1526, and the modulated uplink signal1502 _(U) is transmitted from port 1 to port 2 of end point opticalcirculator 1526.

In the exemplary embodiment of system 1500, to achieve the full benefitof the bidirectional full duplex scheme, coherent modulation is alsoimplemented for the upstream transmission. The bidirectional coherentsignals provide significantly higher optical signal to noise ratio(OSNR) sensitivity, and also higher tolerance to the impairments fromspontaneous Rayleigh backscattering, in comparison with non-coherent,intensity-modulated systems. Additionally, according to theconfiguration of system 1500, the threshold of the stimulated Brillouinscattering (SBS) nonlinear effect is much higher because of the natureof phase-modulated signals on the reduction of optical carrier power andthe increase of effective linewidth.

FIG. 16 is a schematic illustration of an exemplary PON system 1600implementing homodyne detection in a full duplex operation. In anexemplary embodiment, system 1600 represents a CPON configured toimplement COIL-based homodyne detection, in a substantially similarmanner to system 1500, FIG. 15, and similarly includes a downlink signal1602 _(D) from a high quality parent/master laser source, transmitted asa downstream coherent wavelength λ_(D), to an ONU 1604 over an opticalfiber 1606, and further includes a slave laser 1608, a first splitter1610, an optical filter 1612, an ONU optical circulator 1614, a secondsplitter 1616, a homodyne detection unit/receiver 1618, an uplinkmodulator 1620 configured to output a modulated upstream wavelengthλ_(U) from an injection locked carrier signal 1622, a hub opticalcirculator 1624 disposed at the end of fiber 1606 that couples with theheadend/hub (not shown in FIG. 16), and an end user optical circulator1626 disposed at the other end of fiber 1606 that couples with ONU 1604.

Different from system 1500 though, system 1600 further includes asemiconductor optical amplifier (SOA) 1628 disposed at the location ofONU 1604. In the exemplary embodiment, SOA 1628 is disposed along thepath of modulated upstream wavelength λ_(U) between uplink modulator1620 and end user optical circulator 1626. In an embodiment, SOA mayhave a similar structure to an FP laser diode (e.g., slave laser 1608),but will not function as a laser source. Although SOAs generally havehigher noise than an EDFA, the low noise output achieved by cloningslave laser 1608 to perform as the master laser enables system 1600 toimplement SOA 1628 in place of a traditional EDFA because noise is muchless of a factor in this particular COIL-based full duplex bidirectionalcoherent system.

FIG. 17 is a schematic illustration of an exemplary dual-fiber PONsystem 1700 implementing homodyne detection. In an exemplary embodiment,system 1700 represents a CPON configured to implement COIL-basedhomodyne detection, in a substantially similar manner to system 1400,FIG. 14, and similarly includes a downlink signal 1702 _(D) from a highquality parent/master laser source, which is transmitted as a downstreamcoherent wavelength λ_(D) to an ONU 1704 over a dedicated downstreamoptical fiber 1706 _(D). ONU 1704 includes a slave laser 1708, a firstsplitter 1710, an optical filter 1712, an optical circulator 1714, asecond splitter 1716, a homodyne detection unit/receiver 1718, and anuplink modulator 1720 configured to output a modulated upstreamwavelength λ_(U) from an injection locked carrier signal 1722.

In exemplary operation, system 1700 performs similarly to system 1400with respect to the reception and processing of downlink signal 1702_(D) by ONU 1704, and downstream coherent wavelength λ_(D) again mayserve as downlink signal 1702 _(D) for both the master source and theLO. In this example, different from the embodiment depicted in FIG. 14,system 1700 further includes a dedicated upstream optical fiber 1706_(U) for optical transport of modulated upstream wavelength λ_(U) (e.g.,in a non-full duplex operation).

FIG. 18 depicts an exemplary linewidth acquisition effect 1800 forlow-cost laser 1802 (e.g., FP laser 1210, FIG. 12C). In an exemplaryembodiment, laser 1802 is configured to operate as one or more of theslave lasers described among the several embodiments, above. Inexemplary operation, a longitudinal mode 1804 in a laser cavity 1806 oflaser 1802 is predominately excited by reception, from an opticalcirculator 1808, of a remote narrow linewidth seed signal 1810 from adownstream master/parent laser source (e.g., ECL laser 1202, FIG. 12C,not shown in FIG. 18). Because only the single longitudinal mode 1804 oflaser cavity 806 is excited, a separate port of optical circulator 1808outputs an upstream (modulated, in the examples above) signal 1812.Thus, despite the fact that the general quality of low-cost laser 1802is such that an output signal thereof would be expected to conform tofrequency distribution curve 1814, according to the advantageousinjection looking techniques described herein, output upstream signal1812 will instead conform to the narrow, high-quality frequencydistribution mode 1816 that substantially corresponds to the linewidthacquired from the remote (e.g., at the hub) master/parent laser source.

The advantageous properties of these techniques are described above withrespect to homodyne detection configurations. As described furtherbelow, these innovative systems and methods are also advantageouslyapplicable to WDM-PON architectures, as well as heterodyneconfigurations of PON systems.

FIG. 19 is a schematic illustration of an exemplary PON architecture1900 implementing WDM. In an exemplary embodiment, architecture 1900includes a hub portion 1902 and an end user portion 1904 communicativelycoupled over a transport medium 1906 (e.g., a single mode fiber, orSMF). A portion 1902 includes a plurality (e.g., 1-N) of hubtransceivers 1908, and end user portion 1904 includes a plurality (e.g.,also 1-N) of end user transceivers 1910. In an embodiment, hub portion1902 of architecture 1900 is configured to receive a master input lasersource 1912 (e.g., a high-quality ECL laser source) at an optical combgenerator 1914, which together effectively create an optical comb sourcehaving an intrinsic property of generating a plurality of simultaneouslow-linewidth channels of wavelengths λ_(1-N) having easily controlledspacing for master laser source 1912.

In an exemplary embodiment, the particular configuration of optical combgenerator 1914 may be according to one or more of a mode-locked laser,an electro-optic modulation scheme, or a gain-switched laser, inaccordance with the specific technical parameters of architecture 1900.In an embodiment, optical comb generator 1914 may further include, as anintegral or separate component, an EDFA (not separately shown) foramplifying and enhancing the optical power of frequency tones (e.g., 32tones, 64 tones) generated from optical comb generator 1914.

The generated wavelengths λ_(1-N) may then be demultiplexed by a firstdemultiplexer/demultiplexer 1916 for individual transport to, andprocessing by, each of hub transceivers 1908. The demultiplexedwavelengths may represent multiple individual CW channels (e.g., 1-N),or may represent a plurality of individual CW source pairs of phasesynchronized coherent tones (e.g., multiple separate individual phasedsynchronized coherent tone source pairs 166, FIG. 1). The respectiveoutputs of hub transceivers 1908 are then combined at a seconddemultiplexer/demultiplexer 1918 for simultaneous transport over fiber1906, after which the combined wavelengths may be separated by a thirddemultiplexer/demultiplexer 1920 for individual reception by respectiveend user transceivers 1910. In this example, master laser source 1912 isseparate from optical comb generator 1914. In other embodiments, masterlaser source 1912 an optical comb generator 1914 may be integrated as asingle unit or apparatus.

Further to the exemplary embodiment, and similar to the severalarchitectures described above, each hub transceiver 1908 may include oneor more of an injection locked hub slave laser 1922, a first hub opticalcirculator 1924, a hub splitter 1926, a hub modulator 1928, a second huboptical circulator 1930, and a hub homodyne detection unit 1932 (e.g., ahomodyne receiver). First hub optical circulator 1924 is, for example,configured to receive the respective narrow CW or tone λ, and inject thelinewidth properties thereof into hub slave laser 1922. Second huboptical circulator 1930 couples hub transceiver 1908 with fiber 1906 byway of multiplexer/demultiplexer 1918.

In a similar manner, each end user transceiver 1910 may include one ormore of a first end user optical circulator 1934, a first end usersplitter 1936, an optical filter 1938 (e.g., a bandpass filter, or BPF),a second end user optical circulator 1940, an injection locked end userslave laser 1942, a second end user splitter 1944, an end user homodynedetection unit 1946, and an end user modulator 1948. Operation of eachend user transceiver 1910 may then be performed according to one or moreof the principles and techniques described above for the respectivetransceivers depicted in FIGS. 14-17 (e.g., ONUs 1404, 1504, 1604, 1708,respectively). Further to the embodiments described above, architecture1900 demonstrates how the injection looking techniques of the presentsystems and methods may be further extended to individual transceivers(e.g., hub transceivers 1908) at the hub location, such as in the caseof a PON implementing a separate laser source for each individualchannel received from the master seed comb source.

FIG. 20 is a schematic illustration of an exemplary PON system 2000implementing heterodyne detection. System 2000 is similar, in manyaspects, to system 1400, FIG. 14, but system 2000 alternativelyconfigured for heterodyne detection, in contrast to the homodynedetection configuration system 1400. In an exemplary embodiment, system2000 and also represents a CPON (i.e., primarily the downlink portionthereof) configured to implement COIL-based heterodyne detection, andincludes a downstream transmission 2002 from a high qualityparent/master laser source, which is transmitted to an ONU 2004 as adownstream coherent wavelength λ (containing both λ_(D) and λ_(U)) overan optical fiber 2006.

In an embodiment, ONU 2004 includes a slave laser 2008 and one or moreof an optical filter 2010, a heterodyne detection unit 2012 (e.g., adownlink heterodyne coherent receiver), a splitter 2014, an opticalcirculator 2016, and an uplink modulator 2018 configured to output amodulated signal of wavelength λ_(U). In the exemplary embodiment,system 2000 may be similarly implemented at the end user location of oneor more of the respective systems described above (i.e., whereheterodyne detection is desired). Downstream transmission 2002 may, forexample, include both a master CW laser source and a plurality ofdownstream signals λ on two separate wavelengths generated at centraloffice or hub location. Downstream transmission 2002 therefore serves toprovide both remote LO and master source delivery to ONU 2004.

In exemplary operation of system 2000, for the downlink, the receivedtransmission 2002 is separated by optical filter 2010 with the signalfrom one arm thereof (i.e., λ_(D)) fed directly to downlink heterodynedetection unit 2012, and the signal from the other arm (i.e., λ_(U))split by splitter 2014 (e.g., a 3-dB splitter), with one split returnedheterodyne detection unit/coherent receiver 2012 as the LO, and theother split injected into the low-cost laser source of slave laser 2008by way of optical circulator 2016. After injection locking with slavelaser 2008, the newly injected signal (i.e., modulated λ_(U)) is sent touplink modulator 2018 to provide the uplink optical source, which may becommunicated upstream by way of a full duplex coherent opticalconnection schemes using an optical circulator (e.g., FIGS. 21-23,below), or realized by a dedicated separate, second fiber (e.g., FIG.24, below).

Similar to system 1400, FIG. 14, in the case of asymmetrical modulation,the upstream optical source of system 2000 may employ an intensitymodulation scheme, such as NRZ-OOK. In an alternative embodiment,injection locking may be performed prior to splitting by splitter 2014(e.g., described further below with respect to FIG. 22), with one armtherefrom used for the downlink LO, and the other arm used for theuplink CW source (i.e., modulated λ_(U)).

According to the advantageous configuration of system 2000, acentralized comb light source at the OLT (e.g., laser source 1912 andoptical home generator 1914, FIG. 19, not shown in FIG. 20) serves toprovide both master seeding sources and LO signals for both of thedownstream and upstream directions in a heterodyne detectionconfiguration. This advantageous configuration enables a PON system toreuse optical sources, in contrast to intradyne detection techniquesconventionally implemented in long-haul systems. Thus, and similar tothe homodyne detection configurations described above, only onerelatively low-cost slave laser (e.g., an FP laser source) need beemployed at each ONU 2004 to realize the effective performance of thehigh-cost master laser (e.g., an ECL laser source).

FIG. 21 is a schematic illustration of an exemplary PON system 2100implementing heterodyne detection in a full duplex operation. In anexemplary embodiment, system 2100 represents a CPON configured toimplement COIL-based heterodyne detection, in a substantially similarmanner to system 2000, FIG. 20, and includes a downstream transmission2102 _(D) from a high quality parent/master laser source, which istransmitted to an ONU 2104 as a downstream coherent wavelength λ(containing both λ_(D) and λ_(U)) over an optical fiber 2106. ONU 2104similarly includes a slave laser 2108, an optical filter 2110, aheterodyne detection unit 2112 (e.g., a downlink heterodyne coherentreceiver), a splitter 2114, an optical circulator 2116, and an uplinkmodulator 2118 configured to output a modulated, injection locked uplinksignal 2102 _(U).

In exemplary operation, system 2100 performs similarly to system 2000with respect to the reception and processing of downstream transmission2102 _(D) by ONU 2104. Different though, from the embodiment depicted inFIG. 20, system 2100 further includes an end user optical circulator2120 disposed at the end of fiber 2106 that couples with ONU 2104, and ahub optical circulator 2122 disposed at the end of fiber 2106 thatcouples with the headend/hub (not shown in FIG. 20). Apart from theimplementation of heterodyne detection (as opposed to homodynedetection) end user optical circulator 2120 and hub optical circulator2122 are substantially similar in structure and function to end useroptical circulator 1526 and hub optical circulator 1524, FIG. 15,respectively.

FIG. 22 is a schematic illustration of an alternative PON system 2200implementing heterodyne detection in a full duplex operation. In anexemplary embodiment, system 2200 represents a CPON configured toimplement COIL-based heterodyne detection, in a substantially similarmanner to system 2100, FIG. 21, and includes a downstream transmission2202 _(D) from a high quality parent/master laser source, which istransmitted to an ONU 2204 as a downstream coherent wavelength λ(containing both λ_(D) and λ_(U)) over an optical fiber 2206. ONU 2204similarly includes a slave laser 2208, an optical filter 2210, aheterodyne detection unit 2212, a splitter 2214, an optical circulator2216, an uplink modulator 2218 configured to output a modulated,injection locked uplink signal 2202 _(U), an end user optical circulator2220, and a hub optical circulator 2222.

System 2200 differs from system 2100 in regard to the structuralconfiguration of ONU 2204. Specifically, and as described above withrespect to FIG. 20, in ONU 2204, injection locking to slave laser 2208is performed directly from the λ_(U) arm from filter 2210 (i.e., by wayof optical circulator 2216), and prior to reaching splitter 2214, whichthen provides one modulated λ_(U) split to heterodyne detection unit2212 to be used for the downlink LO, and the other modulated λ_(U) splitto uplink modulator 2218 to be used for the uplink CW source of uplinksignal 2202 _(U).

FIG. 23 is a schematic illustration of an exemplary PON system 2300implementing heterodyne detection in a full duplex operation. In anexemplary embodiment, system 2300 represents a CPON configured toimplement COIL-based homodyne detection, in a substantially similarmanner to system 2100, FIG. 21, and similarly includes a downstreamtransmission 2302 _(D) from a high quality parent/master laser source,which is transmitted to an ONU 2304 as a downstream coherent wavelengthλ (containing both λ_(D) and λ_(U)) over an optical fiber 2306. ONU 2304similarly includes a slave laser 2308, an optical filter 2310, aheterodyne detection unit 2312, a splitter 2314, an optical circulator2316, an uplink modulator 2318 configured to output a modulated,injection locked uplink signal 2302 _(U), an end user optical circulator2320, and a hub optical circulator 2322.

Different from system 2100 though, system 2300 further includes an SOA2324 disposed along the path of uplink signal 2302 _(U) between uplinkmodulator 2318 and end user optical circulator 2320. Although notillustrated in FIG. 23, SOA may be similarly disposed in the alternativefull duplex heterodyne configuration depicted in FIG. 22, for example,between uplink modulator 2218 and end user optical circulator 2220.

FIG. 24 is a schematic illustration of an exemplary dual-fiber PONsystem 2400 implementing heterodyne detection. In an exemplaryembodiment, system 2400 represents a CPON configured to implementCOIL-based homodyne detection, in a substantially similar manner tosystem 2000, FIG. 20, and includes a downlink transmission 2402 _(D)from a high quality parent/master laser source, which is transmitted toan ONU 2404 over a dedicated downstream optical fiber 2406 _(D). ONU2404 includes a slave laser 2408, an optical filter 2410, a heterodynedetection unit 2412, a splitter 2414, an optical circulator 2416, and anuplink modulator 2418 configured to output a modulated, injection lockeduplink signal 2402 _(U).

In exemplary operation, system 2400 performs similarly to system 2000with respect to the reception and processing of downlink transmission2402 _(D) by ONU 2404. In this example though, and different from theembodiment depicted in FIG. 20, system 2400 further includes a dedicatedupstream optical fiber 2406 _(U) for optical transport of modulated,injection locked uplink signal 2402 _(U) (e.g., in a non-full duplexoperation).

According to the innovative systems and methods described herein, theproblems experienced with conventional direct detection PONs (i.e., poorreceiver sensitivity, power fading due to chromatic dispersion at highsymbol rates and long transmission distances, bandwidth- andpower-inefficient modulation, etc.) are overcome. The CPONs of thepresent systems and methods are capable of fully realizing the benefitsof coherent optical technology (i.e., frequency selectivity, lineardetection, superior receiver sensitivity, etc.) but in a significantlymore cost-effective solution than has been previously available forcoherent systems. That is, the present COIL-based architecturalconfigurations for a P2MP network effectively extend the reach and splitratio to multiple end-users, and residential home subscribers inparticular, at approximately the present cost of conventional directdetection systems, but also at performance levels comparable to presentcoherent P2P links.

Optical Injected Laser for Coherent Communication

At present, data demand is rapidly increasing at an exponential pace,driven by bandwidth-intensive applications such as “big data,” Cloudtechnology, the Internet of Things (IoT0, video, and augmentedreality/virtual reality (AR/VR). For these reasons, access bandwidthrequirements for delivering such high-speed data and video services isexpected to grow as described above. PON access technologies have thusbecome dominant architectures for meeting end user high capacity demand.Future PONs seek to provide higher per-subscriber data rates and widercoverage of services and applications, and service providers seekminimization of CAPEX and OPEX while increasing reconfigurablecapability for scalable solutions.

As described above, DSPs are often fabricated using CMOS technology and,with recent advancements to sub-10 nm CMOS technology, future DSPs areexpected to support the higher data rates under growing demand, but witha reduced footprint and power dissipation. The overall cost ofconventional systems is dominated by optical and electrical componentssuch as low-linewidth tunable lasers source for transmitter and localoscillator, as well as high speed balanced detectors. For mass adoptionof coherent access network technology in the P2MP paradigm, furtherinnovations to coherent optical components are needed to significantlyreduce the cost, and thus increase the affordability, to an end user'shome.

The embodiments above describe COIL technology and applications thatenable low-quality lasers/slave lasers to perform as high-quality lasersby injection locking from a high-quality laser/master laser. Theinjection locked slave laser thus performs like the high-quality laserin a coherent transmission systems. The embodiments above describe howCOIL may be achieved by injecting a single frequency laser source fromthe master laser into the laser resonator of the slave laser, such aswith multi-longitudinal modes. Through the implementation of COIL in acoherent fiber communication system (e.g., CPON), a single high-qualitylaser may be utilized to drive many slave lasers, and these slave laserswill behave like the single high-quality master laser upon locking tothe master laser. By these COIL techniques, the spectral quality of themaster laser will be effectively cloned into many low-quality, low-costslave lasers, thereby dramatically reducing the overall system cost byavoiding the deployment of high-quality, high-cost master lasers in eachcoherent transceiver (e.g., P2P systems).

However, conventional injection locking techniques are known toexperience degradation of the injection efficiency of the laser due todetuning, and thus also penalties to the side mode suppression ratio(SMSR). A high maximum SMSR (e.g., for fine tuning) may be significantlyreduced by detuning. Conventional frequency locking ranges of a coherentlight source are often less than half of the FP mode spacing, andtherefore, the injected input wavelength of such conventional systemsare only able to tolerate a small variation gap before locking is lost.One significant factor impacting the SMSR of an injection-locked FPlaser is frequency misalignment about the master wavelength. Thefrequency misalignment is due to the asymmetry of the locking range,resulting in an injection locking detuning range that is not equal onboth sides of the frequency spectrum about the master wavelength.Accordingly, improved techniques are desirable for controlling theinjection locking capability with respect to the detuning range.

The present systems and methods therefore provide innovative structuresand processes for COIL-based, low-cost transmitters that may beimplemented with one or more of the embodiments described above. In anexemplary embodiment, the COIL-based transmitter described hereinincludes a master seed laser source with a single longitudinal lasingmode, an input data stream, and a laser injected modulator including atleast one slave laser having a resonator frequency that is injectionlocked to the single longitudinal modal frequency of the master seedlaser source. For example, as described above with respect to FIG. 1,laser injected modulator 138 of downstream transmitter 126 is configuredto receive high quality source signal 116 from an master seed lasersource 118 and an input data stream, and output laser modulated datastream 172. The present COIL transmitter is therefore particularlyuseful for implementation in such coherent-DWDM-PON architectures.

The COIL transmitter embodiments described further herein thus furtherimprove upon the innovative architectures described above by providinginnovative structures and implementation methods that may be employedas, or in place of, either or both of downstream transmitter 126 andupstream transmitter 160. In an exemplary embodiment, both oftransmitters 126 and 160 have similar internal architectures andoperations. Accordingly, for ease of explanation and not in a limitingsense, the following description refers generally to a COIL-basedtransmitter, or COIL transmitter, that may be employed as either anupstream or downstream transmitter in one or more of the embodimentsabove. The architecture of a COIL transmitter for a fiber communicationnetwork is described further below with respect to FIG. 25.

FIG. 25 is a schematic illustration of an exemplary COIL transmittersystem 2500. In an embodiment, COIL transmitter system 2500 includes amaster laser source 2502, a slave laser 2504, a polarization controller2506, an optical circulator 2508 (e.g., a three-port circulator), amodulator 2510, and a smart controller 2512 configured to manage andoptimize performance of COIL transmitter system 2500. In an exemplaryembodiment, system 2500 represents a block architecture for COILtransmission, and master laser source 2502 may, for example, be locatedat the OLT, optical hub, or central office (CO) as the optical coherentsource operating throughout system 2500 (e.g., master seed laser source118, FIGS. 1-7).

Slave laser 2504, optical circulator 2508, and modulator 2510, on theother hand, may be separately located from master laser source 2502 in adiscrete, compact transmission device that is still otherwise disposedin or proximate to the OLT/hub/CO (e.g., downstream transmitters 126,200, 300, 400, 500, FIGS. 1, 2, 3, 4, 5, respectively). Alternatively,or additionally, slave laser 2504, optical circulator 2508, andmodulator 2510, may be remotely located from master laser source 2502 ina discrete, compact transmission device (e.g., upstream transmitters160, 600, FIGS. 1, 6, respectively) at the opposite end of an opticaltransport medium (e.g., fibers 108, 110) within an ONU of an end user106, FIGS. 1, 6.

In exemplary operation of system 2500, the ONU thus is enabled to act asthe device that functions as the service provider endpoint of a PON(e.g., end user 106). In an embodiment, polarization controller 2506 maybe utilized to align the polarization of master laser source 2502 withslave laser 2504 for maximum internal injection coupling efficiency. Infurther operation of transmission system 2500, optical circulator 2508is used to route master laser source 2502 into the resonance cavity ofthe slave laser 2504 (e.g., through Port 1). Thus, the same opticalcirculator 2508 may further serve to route the optical output power ofthe injection locked slave laser (e.g., Port 2) into the input port(e.g., Port 3), and thus to optical modulator 2510. The subsequentcoherent locked laser light may then be modulated using modulator 2510(e.g., which may be an external modulator) to convert a data stream 2514from electrical signals to optical signals for fiber opticaltransmission as modulated data 2516.

In the exemplary embodiment, the architecture of COIL transmitter system2500 may be fully integrated within the greater architecture of fibercommunication system 100, FIGS. 1-7. Master laser source 2502 may, forexample, include a frequency laser comb source (e.g., optical frequencycomb generator 114), configured to generate multiple coherent tones,and/or phase-synchronized coherent tone pairs, at a defined frequencyspacing. In some embodiments, master laser source 2502 includes one ormore of a tunable ECL, a WDM laser array(s), mode locked lasers, orgain-switched lasers. In this embodiment, master laser source 2502represents a high spectral quality, single frequency, or WDM lasersource. However, these types of master laser sources are known to bevery complex and expensive in comparison with other laser types.

In the exemplary embodiment, an ideal master laser source 2502 has lowspectral linewidth, low phase noise, well defined frequency spacing,high E/O quantum efficiency, high operation stability, and high outputpower. The channel spacing of master laser source 2502 may include,without limitation, one or more of 100 GHz, 50 GHz, 25 GHz, or 12.5 GHz,based on the signal bandwidth occupancy. The generated frequencytones/phase synchronized coherent tone pairs (e.g., 32 tones, 64 tones)may then be amplified by an EDFA (e.g., EDFA 122) to enhance opticalpower, and then further demultiplexed into multiple separate individualcoherent sources used as LO signals, and/or for coherent injectionlocking.

In further operation of system 2500, placement of the optical frequencycomb source (e.g., comb generator 114) at the OLT, optical hub, or CO,provides a remote source for generating multiple coherent tones as theLO and seed signal sources for both downstream and upstream directions.This centralized master laser source thus enables the overall system toemploy relatively compact physical devices for the transmissionsubsystem arrangement of system 2500, to provide multi-wavelengthemission capability for each individual COIL transmitter system 2500 ina significantly simplified, elegant, lower cost design that demonstratessignificantly increased reliability. Furthermore, the implementation ofa central comb source provides further advantages that are not readilyprovided by discrete lasers, such as simultaneous control of multiplesources.

As shown in FIGS. 1-7, a single master laser source 118 provides asingle “pure” high quality source signal 116 capable of generatingmultiple separate individual phased synchronized coherent tone sourcepairs 166, where each tone source pair for a single channel is processedby its own pair of respective transmitter devices (e.g., transmitters126, 160). Accordingly, a single fiber access network system may includea single master laser source 2502 for multiple COIL transmitter systems2500. Each such COIL transmitter system 2500 may have its own uniqueslave laser 2504 and modulator 2510, but all of the multiple COILtransmitter systems 2500 may share the single master laser source 2502,but be configured specifically for a single channel/channel pairgenerated therefrom. In the example illustrated in FIG. 1, Ch 1 and Ch1′ are a representative example of a source pair 166 of phasesynchronized coherent tone wavelengths, with Ch 1 used as a downstreamseed and upstream LO, while Ch 1′ is used as a downstream LO andupstream seed.

Conventional transmitters used in long-haul transport utilizehigh-quality lasers and optical modulators to convert electrical datainto optical signals. In comparison with the present COIL transmitterarchitecture, the conventional long-haul transmitter may be consideredto have a relatively simple design. Nevertheless, despite this relativesimplicity, the conventional transmitters are considerably moreexpensive to implement, due to the high cost of the using a dedicatedmaster laser source for each modulator. In the access network paradigm,to be economically viable, the cost of optical components should beconsiderably lower than components used in the long-haul network, andparticularly within the realm of coherent WDM PON. The present COILtransmitter embodiments thus enable significant reductions to thetransmitter cost, and without sacrificing quality of the modulator lasersource. By sharing a high-quality master laser source among manytransmitters using COIL, the present low-cost local slave lasers inheritphase characteristics from the master laser, including frequency andlinewidth. According to the COIL transmission techniques describedherein, a fiber communication network is able to operate a single (orvery few) high-quality master laser sources to drive many times morelow-cost coherent transmitters for each single master laser source withnegligible performance penalty, where the multiple driven transmittersare respectively distributed throughout the network at end user's homes,and/or include multiple compact upstream transmitters at the OLT/hub/CO.

Many laser sources have been used as slave laser sources for injectionlocking across a wide spectral range, including multi-longitudinal modeFP lasers and reflective semiconductor optical amplifiers (RSOAs).However, DFB, DBR, and VCSEL lasers are also known to be capable ofbeing injection locked with improved bandwidth and spectral width. Forthe WDM-PON based access network described herein, multimode FP lasersare described for use as the exemplary slave laser, since the gainspectrum occupied by the FP modes is considered wide enough to cover anyWDM-PON channel on a fiber transmission window, and therefore theinnovative concepts of the present embodiments may be illustrated forall of these known laser structures by the description of the FP laseras an exemplary embodiment. The person of ordinary skill in the art willunderstand that this example is thus used for illustrative purposes, andis not intended to be limiting. An injection locked FP laseradditionally demonstrates good phase noise and linewidth properties.Therefore, by optimizing the FP laser resonance cavity length, a single(or a few) types of FP laser are adequate to illustrate the entireoperational spectral range of a WDM-PON network, thereby furtherallowing for a significant reduction in the number of inventory partsand implementation costs.

Exemplary operation of smart controller 2512 within COIL transmissionsystem 2500 is described further below with respect to the followingfigures. In an exemplary embodiment, smart controller 2512 furtherincludes one or more of a master interface 2518 to master laser source2502, a slave interface 2520 to slave laser 2504, and a modulationinterface 2522 to modulator 2510.

FIG. 26 is a graphical illustration depicting a spectral plot 2600 of anoperational multimode FP laser. In an exemplary embodiment, plot 2600illustrates an operational principle of a multimode FP laser including aplurality of resonant modes 2602, and a resonance cavity with gain mediaexhibiting a gain curve 2604. The resonance cavity may, for example, beformed between a front reflection mirror surface and a rear reflectionmirror surface (not shown in FIG. 26). For optimized output powerefficiency, the rear reflection mirror surface typically is providedwith a higher reflectivity than the front reflection mirror surface. Astable resonant mode 2602 is formed in the FP laser when: (i) theinteger of the wavelength is equal to the round trip optical path of thelaser cavity, and (ii) the particular resonant mode 2602 is withinspectral range of the gain media, as represented by gain curve 2604.Mathematically, the laser modal wavelength λ relates to the laserresonance cavity length L according to the equation N*λ=2*L, where N isa positive integer greater than one.

FIG. 27 is a graphical illustration depicting a spectral plot 2700 of aCOIL FP laser. In an exemplary embodiment, plot 2700 illustrates a powercurve 2702 of a COIL FP laser spectrum including a peak frequency 2704locked to a master laser wavelength 2706, that is, locked by the FPlaser under test. Plot 2700 further illustrates a first plurality ofside modes 2708 to the “left” of peak frequency 2704 (i.e., atfrequencies lower than that of peak frequency 2704) and a secondplurality of side modes 2710 to the “right” of peak frequency 2704(i.e., at frequencies greater than peak frequency 2704). Plot 2700 andthus illustrates an important characteristic of FP laser injectionlocking, described above, namely, that a peak power level 2712 of lockedpeak frequency 2704 is significantly greater than relative power levels2714 of the respective side modes 2708, 2710 of the FP laser. Adifference 2716 between peak power level 2712 and side mode power level2714 represents the SMSR of the FP laser, and side modes 2708, 2710 bythus also referred to as “suppressed modes.” As a rule of thumb, an SMSRvalue of 30 dB or higher is regarded as good optical injection locking,in this example (which exhibits an SMSR greater than 40 dB from plot2700). Higher SMSR values enable better transmission distances, and withreduced phase and amplitude noise over the transmission. In practice,the SMSR may be dependent on specific link budget requirements.

FIG. 28 is a graphical illustration depicting a spectral plot 2800 of apartially locked COIL FP laser. Plot 2800 is similar to plot 2700, FIG.27, in several respects, and similarly includes a peak frequency 2802that is the master frequency, but in this example, peak frequency 2802is only partially locked by the FP laser. Plot 2800 further illustratesa first plurality of left side modes 2804, and a second plurality ofright side modes 2806. Accordingly, the SMSR from plot 2800 may be seento not generally exceed approximately 15 dB, and is therefore notconsidered usable for coherent optical communication.

Therefore, a simple comparison of plot 2800 with plot 2700 demonstrateshow a slave FP laser is considered to be sufficiently coherently lockedto the master laser only under certain conditions. If not properlycontrolled, the slave laser side modes might be unlocked (e.g., plot2600), or only partially locked (e.g., plot 2700), rendering theparticular slave laser unusable for the COIL techniques describedherein. Accordingly, the present COIL transmitter systems and methodsadvantageously enable coherent optical fiber communication techniques towork more reliably in the field by locking the optical injection lasersource with operational certainty and managing the ongoing operationwith significantly greater stability.

In an exemplary embodiment, a smart controller (e.g., smart controller2512, FIG. 25) is implemented within the COIL transmitter architecture(e.g., COIL transmitter system 2500, FIG. 25) to improve both theinitial COIL locking for the coherent optical communications, as well asthe ongoing stability of the COIL locking under continuous operation ofthe slave laser with respect to be master laser source. In someembodiments, the smart controller operates according to guidelines foroptimizing the optical injection locking process to achieve highestoptical output power and highest SMSR for the system, and also toexpedite a fast and stable locking condition. In an exemplaryembodiment, the smart controller further includes advanced sensing andcontrol interfaces to both the master laser (e.g., master interface2518, FIG. 25) and the slave laser (e.g., slave interface 2520, FIG.25), as well as an interface with the coherent optical modulator (e.g.,modulation interface 2522, FIG. 25) configured to provide optimizedpower and bias control. In the exemplary embodiment illustrated in FIG.25, smart controller 2512 is configured to manage and receive outputoptical power and wavelength information (e.g., λ & I) through masterinterface 2518 from master laser 2502, and the received power andwavelength information (e.g., T & I) may then be used to control slavelaser 2504, through slave interface 2520, to be injection locked to themaster wavelength.

FIG. 29 is a graphical illustration depicting a comparative plot 2900 ofslave laser frequency against master laser power. In the exemplaryembodiment depicted in FIG. 29, comparative plot 2900 illustrates theinjection locking frequency range of the slave laser (e.g., slave laser2504, FIG. 25) against the root squared input power of the master laser(e.g., master laser source 2502, FIG. 25). More particularly, thevertical axis of plot 2900 represents a detuning range 2902 of the slavelaser injection locking frequency of an FP laser at room temperature,and the horizontal axis plot 2900 represents square root of the masterlaser input optical power.

In the embodiment illustrated in FIG. 29, frequency detuning range 2902is defined by first locking a side mode of the FP slave laser with themaster laser frequency, and then the FP side mode is detuned infrequency with respect to the master frequency until the particular FPside mode out of injection locking, or has an SMSR less than 30 dB. Asdescribed above, a key attribute of injection locking is that thedetuning frequency range is asymmetrical with respect to the centerfrequency of the FP side mode. If the master laser frequency is set at acenter of the FP side mode, injection locking may be achieved (describedfurther below with respect to FIG. 30). However, if the master laserfrequency is set slightly on the longer wavelength (i.e., lowerfrequency) side (e.g., first side modes 2708, FIG. 27, 2804, FIG. 28),it will have more tolerance to detuning.

As further illustrated in FIG. 29, a first subplot 2904 representsred-shift measured data, a second subplot 2906 represents measuredblue-shift data, and a third subplot 2908 represents the optimizedcenter locking frequency over the square root of injection power. Afourth subplot 2910 and a fifth subplot 2912 represent curve fitted dataof first subplot 2904 and second subplot 2906, respectively. Asindicated by comparative plot 2900, the red-shift approach exhibits agreater tolerance to maintain the locking state in a larger frequencyrange in comparison with the blue-shift approach. This phenomenon islabeled “red-shift” in optical injection locking theory due to thelinewidth enhancement factor of the master laser induced carriervariation which will induce the gain change of the slave laser to longerwavelengths.

Therefore, by considering the COIL information against the input powerinformation, very valuable additional information is provided thatenables the enhanced slave laser injection locking control describedherein. For example, in the case where the master laser frequency isplaced right in the middle of a corresponding FP side mode center,locking may be effectively realized, but the stability of this lockingwill not be optimized, since this locking frequency will not becentered/in the middle of detuning range 2902 (e.g., along optimizedthird subplot 2908). Therefore, as illustrated in plot 2900, when themaster laser output power is increased (horizontal axis), detuning range2902 of the slave laser locking also increases. Accordingly, plot 2900demonstrates that the locking range and stability of the COIL slavelaser may be improved by increasing the master laser power.

FIG. 30 is a graphical illustration depicting an optimization scheme3000 for injection locking control. In an exemplary embodiment,optimization scheme 3000 represents an injection locking controlillustration diagram useful for optimized control injection locking ofFP slave laser modes 3002 with respect to a master wavelength 3004. Asillustrated in FIG. 30, the center wavelength of master wavelength 3004is denoted as λ_(ML), and λ_(SLn) denotes the center wavelength of then^(th) side mode of FP slave laser modes 3002. Accordingly, a difference3006 between the respective center wavelengths of master wavelength 3004and the n^(th) side mode 3002 _(n) may be represented according toΔλ=λ_(ML)−λ_(SLn), where Δλ represents the wavelength offset between thecenter wavelength λ_(ML) of the master laser/master wavelength 3004 andthe center wavelength λ_(Sln) of the n^(th) side mode 3002 _(n) of theslave laser.

Referring back to FIG. 25, in an exemplary embodiment, smart controller2512 includes a memory capable of storing information and algorithmsrelevant to the injection locking physics and characteristics.Configured with such additional capability, smart controller effectivelybecomes a “brain” of COIL transmitter system 2500. In some embodiments,smart controller 2512 is further configured to be capable of sensing ormeasuring the laser current in real-time, and also the temperature andoutput power, through one or more control data acquisition interfaces(e.g., interfaces 2518, 2520, 2522). In at least one embodiment, smartcontroller 2512 is configured to control master laser source 2502 byinitially setting, and later real-time adjustment in some cases, theoptical power and optical frequency.

Similarly, smart controller 2512 may be further configured to alsocontrol slave laser 2504 by setting the bias current and the outputpower thereof. In an exemplary embodiment, smart controller 2512 isconfigured to control slave laser 2504 by managing the slave laserjunction temperature, such as by setting the temperature controller (notshown) of slave laser 2504. In at least some embodiments, opticalmodulator 2510 may also be managed by smart controller 2512, such thatmodulator 2510 is provided with optimized power and bias for theincoming coherently locked FP slave laser output.

FIG. 31 is a flow chart diagram of an exemplary control process 3100 forsmart controller 2512, FIG. 25. In an exemplary embodiment, process 3100represents a smart controller process flow for initiating, configuring,and operating COIL transmitter system 2500, FIG. 25. In someembodiments, process 3100 may be further optionally implemented withrespect to one or more embodiments of fiber communication system 100,FIGS. 1-7, including execution in a complementary, non-exclusionaryfashion with respect to either or both of process 800, FIG. 8, andprocess 900, FIG. 9. Except where described to the contrary, individualsteps of process 3100 may be performed in the order described, adifferent order, and/or two or more of several steps may be performedsimultaneously.

In the exemplary embodiment, process 3100 begins at step 3102, in whichoperation of smart controller 2512 begins. In step 3104, the masterlaser (e.g., master laser source 2502) is powered on. In an exemplaryembodiment of step 3104, smart controller 2512 turns on master lasersource 2502 by setting a predefined wavelength and optical power level.In step 3106, smart controller 2512 is configured to calculate the drivecurrent and temperature needed for slave laser 2504 to be successfullyinjection locked to the known master laser source 2502. In an exemplaryembodiment of step 3106, smart controller 2512 obtains the drive currentby a lookup operation in memory, or may obtain the drive current from aprecision current source and the temperature by direct or indirectsensing capability.

In step 3108, smart controller 2512 transmits operational commands to adriver of slave laser 2504, as well as a temperature controller thereof(not shown). In an exemplary embodiment of step 3108, temperaturecontrol may be provided using a thermal electrical ceramic (TEC) module,or using a simplified thermal resistive heater element may beimplemented for temperature control. In at least one embodiment of step3108, temperature control is provided by smart controller 2512 using athermistor (not shown) co-packaged near the FP laser chip of slave laser2504 to accurately control the FP chip junction temperature within onepercent of a Celsius degree or better.

In the exemplary embodiment, smart controller 2512 is configured tomaintain in memory a detailed knowledge of the FP laser detuning range,as well as optimized locking temperatures and currents for theparticular FP slave laser sought to be controlled. In an exemplaryembodiment, such detailed information and/or algorithms may be stored ina lookup table, in the smart controller memory, in an EEPROM, etc. In anexemplary embodiment, smart controller is, or includes, a server, amicro-processor, or an ASIC.

In step 3110, smart controller 2512 polls slave laser 2504 for thelocking status of slave laser 2504. Step 3112 is a decision step. If, instep 3112, smart controller 2512 determines that slave laser 2504 hasnot achieved locking to master laser source 2502, process 3100 returnsto step 3106. If, however, process 3100 determines in step 3112 thatslave laser 2504 has achieved injection locking, process 3100 proceedsto step 3114. In step 3114, smart controller 2512 configured to verifythe power and bias operation condition of modulator 2510 (e.g., usingmodulation interface 2522). In an exemplary embodiment of step 3114,smart controller 2512 is further configured to adjust, in real time, themodulator power and bias operation condition as desired or needed.

In step 3116, smart controller 2512 is configured to initialize theoptical transmission, turn on modulator 2510, and enable communicationof data stream 2514, which is then output from modulator 2510 asmodulated data 2516. In step 3118, process 3100 is configured to wait,or hold, for a predetermined period of time (e.g., 0-n seconds), beforereturning to step 3104, thereby creating a continuous monitoring loopthat is capable of adjusting and controlling in real-time the severallaser parameters relevant to successfully initiating and maintaininginjection locking of slave laser 2504 to master laser source 2502. In anexemplary embodiment, the continuous loop of process 3100 will run aslong as smart controller 2512 is powered on.

According to the exemplary systems and methods described herein, aninnovative optical injection locking-based coherent optical transmitteris provided for coherent fiber optical network communications. Thepresent transmitter includes a master laser capable of providing singlechannel or multi-channel low-linewidth frequency channel(s) as thecentral laser source for the communication system. The transmitter mayfurther include a slave laser having multi-longitudinal modes which maybe coherent optical injection locked (COIL) to the master laserfrequency of the master laser.

In some embodiments, a COIL-based transmitter system includes apolarization controller between the master laser and the slave laser tomaximize the injection locking efficiency, and an optical circular maybe employed to inject the master laser into the cavity of the slavelaser. The output power from the locked slave laser may be transmittedalong the same fiber from which the master laser is injected to theslave laser cavity, but alternatively routed to an optical modulator. Inthe exemplary embodiment, operation of the COIL-based transmitter systemis managed by a smart controller having a memory, and/or programmed withsmart algorithm, such that the master laser and the slave laser may bereliably placed, and then maintained, in a locking state.

In some embodiments, the smart controller represents an advanced controldevice having improved communication, sensing, and control capabilities.The smart controller may, for example, include or be a server, amicroprocessor, or an ASIC. The smart controller may further include amemory or EEPROM capable of storing detailed tables of slave laser or FPlocking conditions with respect to the injected master laser opticalpower, and which may further include detailed information regardingrelevant slave laser drive currents and junction temperatures.

In exemplary embodiments of the systems and methods described herein,the master laser communicates with the smart controller such thatoptical frequency and optical power is known to the controller. Thesmart controller may further include one or more interfaces capable ofcommunication with one or more respective components of the COILtransmitter system. The master laser may, for example, include or be afrequency comb, a tunable laser, a WDM array, and/or other types oflasers capable of providing low-linewidth, and low phase noise opticaltone or tones. The master laser may thus advantageously communicate withthe smart controller, and in some cases, may be controlled by the smartcontroller.

Similarly the slave laser may also communicate with the smart controllerand be controlled by the smart controller. The slave laser may thereforeinclude or be a multi-longitudinal FP laser, a VCSEL, a DFB, or anothertype of laser capable of being injection locked to provide a single toneCOIL source. The slave laser may optimally be driven by high precisioncurrent source of the master laser to provide injection locking byaligning an FP side mode with the master laser frequency. Alternatively,or supplementally, the slave laser may be driven by high resolutiontemperature controller such that injection locking is provided byaligning the FP side mode with the master laser frequency. Thetemperature controller may be or include one or more of a TEC componentclose to the FP laser capable of heating or cooling the FP laser, athermal resistor component close to the FP laser capable of heating theFP laser, and a thermistor or temperature sensor placed near the FPlaser to monitor the FP laser junction temperature.

In the exemplary embodiments described herein, the COIL transmittersystem includes optical modulator capable of converting an electricaldata stream into a modulated optical signal using the COIL transmittersystem source. In some embodiments, the bias and power of the opticalmodulator may also be advantageously controlled by the smart controller.The optical modulator may include or be an intensity modulator, a phasemodulator, an intensity-phase modulator, and/or one or more of themodulator embodiments described above.

The present techniques further provide an advantageous processingalgorithm for operating the smart controller manage the transmitterperform, and thereby advantageously ensure that COIL is not onlyachieved, but also maintained in real-time throughout the operationalduration of the smart controller. The smart controller may thus beconfigured to seamlessly integrate with the operation or one or more ofthe hardware component and system structures described above.

The present smart controller may therefore further include one or moresoftware algorithms, or sets of executable computer programming steps,including without limitation instructions to: (i) initiate the masterand slave lasers; (ii) calculate or determine optimal conditions toensure locking of the slave laser; (iii) send drive current and/ortemperature control commands to the slave laser; (iv) poll thetemperature and current of the slave laser; (v) set the opticalmodulator power and bias for data conversion; (vi) start modulatoroperation once injection locking is stabilized; and (vi) instruct orauthorize the optical modulator to convert electrical data to modulatedoptical signals for communication over an optical fiber transportmedium. The present systems and methods are therefore of particularutility in the paradigm of coherent communications systems, includingwithout limitation P2P coherent transmission systems, P2MP coherent PONsystems, and/or single wavelength or WDM systems.

Injection-Locking Full-Field Transmitter

As described above, data-driven applications, such as HD videostreaming, mobile Internet, and cloud computing, have led to asignificant growth in the recent overall traffic volume in opticalcommunication networks. Furthermore, the demand for optical transmissionat ultra-high data rates is also increasing for the optical transport,metro, and access network paradigms. These trends, along with a growingbit-per-Hz cost reduction requirement, have led to increasing needs inthe industry for higher-speed optical transmission interfaces and higherspectrum efficiency technologies.

Recent developments in coherent optical technology have provided usefulimprovements for long-haul backbone transport systems. Advancements indigital signal processing (DSP) techniques, as well as signal formats(e.g., QPSK, 8QAM, 16QAM, etc.) utilizing polarization multiplexing,have enabled long-haul links, and metro networks more recently, torealize improve data rates and spectral efficiency. Coherenttechnologies are now being introduced to the access network, such aswith the case of 100G coherent optics in short distance P2P links of theHFC access network. Other wired networks (e.g., PON) are also movingtowards 100G coherent implementations for next generation (NG) access(e.g., NG-PON). However, the lower receiver sensitivity and limitedpower budget of PON systems present considerable challenges to highspeed support using present PON direct detection schemes. The innovativecoherent techniques presented herein though, address and solve theseconventional challenges, rendering the implementation thereofsignificantly future-proof solutions for NG high-speed PON systems. Thepresent systems and methods improve the link power budget, and with muchgreater sensitivity, while also enabling higher-order modulation formatsto increase the data rate and network efficiency.

Cost is another significant obstacle to utilizing coherent optics in theaccess network. Conventionally, the overall system cost of the accessnetwork has been dominated by optical and optic-electronic components,such as low-linewidth tunable laser sources for the transmitter, as wellas local oscillators and advanced modulators. Accordingly, there is aneed in the industry for further innovations to the coherent opticalcomponents of the coherent access network to further reduce the costthereof. Some such architectural innovations are described in greaterdetail above, and in U.S. Pat. No. 9,912,409, all of which areincorporated by reference herein in their entireties. These embodimentsto the present inventors describe COIL techniques that enable the use ofessentially “low-quality” and low-cost lasers (i.e., slave lasers) ashigh performing lasers by injection locking the low-cost laser by ahigh-quality laser (sometimes referred to herein as “master laser”). Theinjection locked slave laser thus behaves like a high-quality masterlaser which can be used in coherent transmission systems to greatlyreduce the overall cost of the entire system.

Particular challenges with respect to injection locking up lasers aredescribed in greater detail above. The present embodiments introduce newand advantageous implementation schemes for injection-locking full-fieldtransmitters that incorporate newer technologies, such that thesedeveloping coherent technologies are able to meet the uniquerequirements of the access environment with cost-efficient approaches.For example, according to the present systems and methods, laserinjection-locking and full-field signal modulation (e.g., as m-QAM orm-PSK) may both be achieved using the low-cost embodiments describedherein. The present techniques are further enabled to achievepolarization multiplexing, such as for dual-polarization advanced signalmodulations. The present embodiments thereby provide an injection-lockedtransmitter using a relatively low-cost configuration of opticalcomponents, but which achieves high spectrum efficiency for an opticalcoherent communication system. In comparison with conventional I/Qmodulators that implement a parallel Mach-Zehnder Modulator (MZM)structure that suffers relatively high insertion and modulation losses,the present systems and methods represent significant reductions to notonly the cost, but also to the modulation loss, while neverthelessincreasing the output power.

The embodiments described further herein provide, for purposes ofillustration, two exemplary system architectural scenarios for multipleinjection-locked transmitters, namely: (i) single-wavelength source usecases; and (ii) multi-wavelength source use cases. The person ofordinary skill in the art though, when reading and comprehending thepresent disclosure, will understand that these exemplary embodiments areprovided by way of example, and not in a limiting sense. Otherarchitectural scenarios may be implemented with respect to the presentembodiments without departing from the scope thereof. The presentsystems and methods are further of particular use in both P2P and P2MPnetworks, and in either network type, a single high-quality master lasermay be cloned and modulated by one or more low-cost slave lasers whichwill exhibit the high-quality performance of the master laser, whilealso carrying the relevant advanced modulation formats. According tothese innovative techniques, the overall network cost is greatly reducedby avoiding the deployment of high-quality, high-cost master lasers andmodulators for each coherent transceiver.

FIG. 32 is a schematic illustration of a COIL system 3200. In anexemplary embodiment, system 3200 includes a COIL transmission subsystem3202 and an external modulator 3204. COIL transmission subsystem 3202includes a master laser 3206, at least one slave laser 3208, and anoptical circulator 3210. In exemplary operation of system 3200, COILtransmission subsystem 3202 functions similarly to the systems andmethods described in the embodiments above, i.e., low-costinjection-locked lasers. Nevertheless, for at least some of theembodiments described above, use of external modulator 3204 may bedesirable to generate higher-order modulation formats, such as QPSK orQAM.

In the embodiment depicted in FIG. 32, external modulator 3204 isdepicted as an MZM external I/Q modulator, which is consideredrelatively expensive in this particular field, and which also is knownto have a high insertion/modulation loss (i.e., greater than 25 dB). Thepresent embodiments though, avoid the need for such expensive externalmodulators by injection locking lasers using full-field transmitters,that is, full-field optical signal modulation. The following techniquesdescribe full-field optical signal modulation using (1) orthogonalmodulation in the Cartesian coordinate plane (e.g., In-phase (I) andQuadrature (Q) modulation), and (2) amplitude and phase modulations inthe Polar coordinate plane (i.e., amplitude and phase being naturallyorthogonal to one another). Accordingly, for the examples describedherein, optical full-field modulation is described with respect tooptical carriers from each modulation dimension being fully coherent,and at the same-frequency, polarization, and locked phase.

FIG. 33 is a graphical illustration depicting an operational principle3300 of a COIL transmitter (e.g., similar to COIL transmission subsystem3202, FIG. 32) implementing I/Q modulation. In an exemplary embodiment,operational principle 3300 implements full-field optical transmitterinjection locking, based on I/Q modulations, in three stages, i.e., afirst stage 3302, a second stage 3304, and a third stage 3306, andwithout requiring any external modulator (e.g., external modulator 3204,FIG. 32).

In first stage 3302, two direct-modulator COIL lasers (sometimesreferred to herein as COILs) are directly modulated with data for I andQ signals. In the example illustrated in FIG. 33, the first COIL laser(COIL 1) is modulated as the I signal, and the second COIL laser (COIL2) is modulated as the Q signal. In second stage 3304, two controlledphase shifters (PSs, not shown in FIG. 33, see FIG. 36, below) utilizedto combine the resultant two optical signals from first stage 3302(i.e., after injection locking and modulation, with 90° phase differencebetween them) to realize a higher-order modulation (16 QAM, in thisexample). However, at second stage 3304, after combining the I/Qsignals, a DC offset exists in combined signal 3308, due to the directmodulation from the respective COIL lasers. Accordingly, in at least oneembodiment of operational principle 3300, at second stage 3304, thephase shifters may be further utilized to obtain an optical carrier 3310having a 180-degree phase shift. Therefore, at third stage 3306,modulated signal 3308 is combined with shifted optical carrier 3310 toachieve a full-field modulation constellation 3312 that is fullycentered about the I/Q axes.

According to this advantageous embodiment, the need or desire for anexternal modulator is eliminated, thus rendering optical systemaccording to operational principle 3300 “external modulator-free.” Asdescribed above, in this exemplary embodiment, to direct-modulator COILlasers are utilized along with two controlled phase shifters keeporthogonality (i.e., first and second stages 3302, 3304, respectively),with the at least one additional stage (i.e., third stage 3306) tocancel the DC offset resulting from the direct modulation of the COILlasers.

FIGS. 34A-B are graphical illustrations depicting an operationalprinciple 3400 of COIL transmitters implementing amplitude modulationand phase modulation, respectively. More particularly, different fromoperational principle 3300, FIG. 33, operational principle 3400 achievesfull-field transmitter injection locking based on amplitude and phasemodulations implemented in the polar coordinate plane over a first stage3402 and a second stage 3404. At first stage 3402, injection locking andamplitude modulation are realized to obtain and amplitude-modulated COILsignal 3406. At second stage 3404, phase modulation is applied toachieve full-field optical modulation of full-field signal 3408. Since,according to this technique, the amplitude and phase modulation remainnaturally orthogonal to each other, and additional (i.e., third) stagefor bias control is unnecessary.

In comparison with conventional modulators based on MZM or Mach-ZehnderInterferometers, which experience generally large insertion andmodulation losses, the present injection-locking laser-based embodimentsdemonstrate still further advantages over these conventional systems, inthat the present systems and methods are capable of providing asignificantly higher output power due to the gain components present inthe lasers. In comparison with operational principle 3300, FIG. 33,operational principle 3400 may be more simply implemented using only oneCOIL, with no bias or phase shift control needed. In some embodiments ofoperational principle 3400 though, an external phase modulator may bedesired.

FIG. 35 is a schematic illustration of an exemplary single-polarizationCOIL transmitter 3500 implementing full-field modulation based on I/Qmodulation. In an exemplary embodiment, FIG. 35 depicts animplementation process flow of transmitter 3500 implementingsingle-polarization injection locking with full-field modulation basedon I/Q modulation (e.g., according to operational principle 3300, FIG.33). In the exemplary embodiment, transmitter 3500 is aninjection-locked full-field optical transmitter that further includes ahigh-quality master laser 3502, which may be used to injection-lockfirst and second low-cost slave lasers 3504, 3506, respectively, (e.g.,multi-longitudinal mode FP lasers or VCSELs). In an exemplaryembodiment, master laser 3502 and slave lasers 3504, 3506 are inoperable communication over a COIL subsystem 3508.

In the exemplary embodiment, COIL subsystem 3508 includes an opticalcirculator 3510 for routing master laser 3502 into the respectiveresonance cavities (not shown) of slave lasers 3504, 3506. COILsubsystem 3508 further includes a first coupler 3512 (Coupler-1)disposed opposite master laser 3502 with respect to optical circulator3510, for splitting the power from the signal of master laser 3502 intoa first path 3514 (Path-1), and a second 3516 (Path-2). In theembodiment depicted in FIG. 35, first slave laser 3504 (S-LD I) isinjection locked to master laser 3502, and further directly-modulated toconvert a first electrical data stream 3518 of electric signals,received from a first electrical data signal source 3520 (electricalI-data, in this example), into a synthesized optical (I) signal. In asimilar manner, second slave laser 3506 (S-LD Q) is also injectionlocked to master laser 3502, and directly-modulated to convert a secondelectrical data stream 3522 of electric signals received from a secondelectrical data signal source 3524 (electrical Q-data, in this example),into a synthesized optical (Q) signal.

The injection-locked modulated optical (I) signal from first slave laser3504 connects with first path 3514, and the injection-locked modulatedoptical (Q) signal from second slave laser 3506 connects with secondmath 3516 through a first phase shifter 3526. In this manner, firstphase shifter 3526 functions to control the phase shift between firstpath 3514 and second path 3516. Accordingly, after modulation, the twoinjection-locked optical laser signals are combined at first coupler3512. Through this advantageous configuration, implementation of firstphase shifter 3526 enables control of the signals to achieve around-trip of 90-degree phase difference between the two respectiveoutputs of injection-locked slave lasers 3504, 3506, thereby achievingfull I/Q modulation between the slave lasers. In this example, a phaseshift of 90-degrees is provided by way of illustration, but not in alimiting sense. The person of ordinary skill in the art understand thatdifferent phase shift values may be employed with respect to the presentembodiments to achieve a synthesized full-field optical signal withdifferent phase and amplitude distributions.

According to the exemplary configuration of transmitter 3500, opticalcirculator 3510 advantageously provides additional functionality toroute the modulated, synthesized full-field optical signal (i.e., thecombination of first and second paths 3514, 3516) out of thetransmitter. A final stage includes a second coupler 3528 (Coupler-2)disposed between master laser 3502 and first coupler 3512, and a thirdcoupler 3530 (Coupler-3) disposed between optical circulator 3510 and atransmitter output 3532. In the exemplary embodiment, a second phaseshifter 3534 (PS-2) is disposed along a third path 3530 (Path-3) betweensecond coupler 3528 and a third coupler 3530, which effectively rendersthird path 3530 into a bypass route around optical circulator 3510between master laser 3502 and transmitter output 3532. This advantageousconfiguration of second phase shifter 3534 thus serves as a shiftedcontrol, taken directly from master laser 3502 (i.e., at second coupler3528), and then combined with the synthesized full-field optical signalfrom optical circulator 3510 (i.e., at third coupler 3530), toeffectively cancel the direct-current (DC) component of the full-fieldsignal. In an exemplary embodiment, second phase shifter 3534 isconfigured for a 180-degree phase shift, which enables (e.g., assumingan appropriate power level from second coupler 3528) the DC component ofthe synthesized full-field optical signal to be fully canceled at/afterthird coupler 3530.

In an exemplary embodiment, first and second phase shifters 3526, 3534are further configured to operate with a fixed phase shift to achieveorthogonal I/Q modulation. For example, first phase shifter 3526 may beconfigured to have a 45-degree one-way phase shift (e.g., along secondpath 3516), and second phase shifter 3534 may be configured to have a180-degree one-way phase shift (e.g., along third path 3536). In someembodiments, one or both of first and second phase shifters 3526, 3534are tunable, and may utilize a bias voltage to achieve arbitrary ordesired phase and amplitude modulations. Transmitter 3500 thereforerepresents a three-stage signal generation unit. In at least oneembodiment, in a case where the injected power of master laser 3502 isconsidered relatively low, a gain component may be desirable to amplifythe power along third path 3536, before or after second phase shifter3534, for carrier cancellation (e.g., using a semiconductor opticalamplifier (SOA), not shown in FIG. 35).

FIG. 36 is a schematic illustration of an alternativesingle-polarization COIL transmitter 3600 implementing full-fieldmodulation based on I/Q modulation. Transmitter 3600 is similar totransmitter 3500, FIG. 35, in several structural and functionalrespects, and also represents an injection-locked full-field opticaltransmitter setup for a high-quality master laser 3602 in communicationwith a COIL subsystem 3604 including a first slave laser 3606 (S-LD I)and a second slave laser 3608 (S-LD Q) injection-locked to master laser3602. Similar to the operation of transmitter 3500, first slave laser3606 is modulated to convert a first electrical data stream 3610 (e.g.,I-data) from a first electrical data signal source 3612 into an optical(I) signal along a first path 3614, and second slave laser 3608 ismodulated to convert a second electrical data stream 3616 (e.g., Q-data)from a second electrical data signal source 3618 into an optical (Q)signal along a second path 3620, along which is disposed a first phaseshifter 3622. In the exemplary embodiment, COIL subsystem 3604 furtherincludes an optical circulator 3624 for routing master laser 3602 intothe respective resonance cavities (not separately shown) of slave lasers3606, 3608.

Transmitter 3600 differs from transmitter 3500 though, in that COILsubsystem 3604 utilizes a single optical coupler 3626 between opticalcirculator 3624 and slave lasers 3606 and 3608 for combining thesynthesized optical signals of first and second paths 3614, 3620.Transmitter 3600 further differs from transmitter 3500 in thattransmitter 3600 includes an injection-locking third slave laser 3628(S-LD C) in operable communication with optical coupler 3626 over athird path 3630. In an exemplary embodiment, a second phase shifter 3632is disposed along third path 3630 between optical coupler 3626 andsecond phase shifter 3632. According to this alternative configuration,only one coupler (e.g., optical coupler 3626) is needed to split theinput master laser power from master laser 3602 into the three separatelanes of first, second, and third paths 3614, 3620, 3630, respectively.

More particularly, in this example, transmitter 3600 applies first path3614 for laser injection-locking and I-data modulation, and second path3620 for laser injection locking and Q-data modulation, with around-trip 90-degree phase shift provided by first phase shifter 3622.According to this configuration, transmitter 3600 is further enabled toapply third path 3630 for not only laser injection locking, but also forDC offset cancellation through utilization of second phase shifter 3622,which may achieve a 180-degree phase shift round-trip, in this example.Thus, a single optical circulator (e.g., optical circulator 3624) maystill be utilized in this alternative configuration to both route themaster laser signal into the respective resonance cavities of slavelasers 3606, 3608, 3628, and also to route the modulated, synthesizedfull-field, combined optical signal therefrom to a transmitter output3634. In some embodiments, implementation of third slave laser 3628 maybe optional according to the input power of master laser 3602, namely,in the case where the input power is above or below a predeterminedthreshold.

Transmitters 3500 and 3600 are described above with respect tosingle-polarization COIL configurations by way of illustration, and notin a limiting sense. For example, the principles described immediatelyabove are fully applicable to dual-polarization transmitters throughimplementation of two sets of each of the single-polarization COILfull-field transmitters in combination with a polarization beam combiner(PBC) to combine the modulated signals on two polarizations to achievepolarization multiplexing, as described further below with respect toFIGS. 37 and 38.

FIG. 37 is a schematic illustration of an exemplary polarizationmultiplexer transmitter 3700 implementing two sets ofsingle-polarization COIL full-field transmitters. More particularly, fora single master laser 3702, transmitter 3700 utilizes two COILsubsystems 3508, FIG. 35, to modulate the respective I/Q data streamsfor each of an X-polarization and a Y-polarization. In the exampledepicted in FIG. 37, the laser signal from master laser 3702 isdistributed to a first COIL subsystem 3508(X) and a second COILsubsystem 3508(Y) by a preliminary coupler 3704. First COIL subsystem3508(X) then synthesizes a single-polarization optical signal 3706(X)with respect to an X-polarization/I-data electrical source 3708(XI) andan X-polarization/Q-data electrical source 3708(XQ) according to theprinciples described above with respect to FIG. 35. Similarly, secondCOIL subsystem 3508(Y) synthesizes a single-polarization optical signal3706(Y) with respect to a Y-polarization/I-data electrical source3708(YI) and a Y-polarization/Q-data electrical source 3708(YQ).Single-polarization optical signals 3706(X), 3706(Y) are then combinedby a PBC 3710 to produce a dual-polarization modulated optical outputsignal 3712. In the example depicted in FIG. 37, PBC 3710 is illustratedto be integrated within transmitter 3700. In some embodiments, PBC 3710may be a separate and discrete component from transmitter 3700.

FIG. 38 is a schematic illustration of an alternative polarizationmultiplexer 3800 transmitter implementing two sets ofsingle-polarization COIL full-field transmitters. More particularly, fora single master laser 3802, transmitter 3800 utilizes two COILsubsystems 3604, FIG. 36, to modulate the respective I/Q data streamsfor each X/Y polarization. In the example depicted in FIG. 38, the lasersignal from master laser 3802 is distributed to a first COIL subsystem3604(X) and a second COIL subsystem 3604(Y) by a preliminary coupler3804. First COIL subsystem 3604(X) synthesizes a single-polarizationoptical signal 3806(X) with respect to an XI data source 3808(XI) and anXQ data source 3808(XQ), and second COIL subsystem 3604(Y) synthesizes asingle-polarization optical signal 3806(Y) with respect to a YI datasource 3808(YI) and a YQ data source 3808(YQ). Single-polarizationoptical signals 3806(X), 3806(Y) are then combined by a PBC 3810 toproduce a dual-polarization modulated optical output signal 3812.

The full-field COIL transmitter techniques described above for I/Qmodulation are also applicable to amplitude and phase modulation byimplementing an external phase modulator, as described further belowwith respect to FIGS. 39 and 40.

FIG. 39 is a schematic illustration of an exemplary single-polarizationCOIL transmitter 3900 implementing full-field modulation based onamplitude and phase modulation. In an exemplary embodiment, FIG. 39depicts an implementation process flow of transmitter 3900 forsingle-polarization injection-locking, with full-field modulation, basedon amplitude and phase, using a high-quality master laser 3902 toinjection-lock a low-cost slave laser 3904 (e.g., FP, VCSEL, etc.) of aCOIL subsystem 3906. Subsystem 3906 further includes an opticalcirculator 3908 for routing laser signal of the master laser 3902 intothe resonance cavity of slave laser 3904, which may thus beinjection-locked and directly-modulated to carry amplitude informationfrom an amplitude data source 3910. Similar to the embodiments describedabove, optical circulator 3908 additionally functions to route aresultant amplitude-modulated optical signal 3912 from slave laser 3904out of transmitter 3900.

Different though, from the above embodiments, amplitude-modulatedoptical signal 3912 is first modulated by a phase modulator 3914 incommunication with a phase a data source 3916 (e.g., polar coordinates),and then output as a full-field modulated optical signal 3918.Accordingly, through utilization of amplitude modulation from slavelaser 3904 and phase modulation from phase modulator 3914, transmitter3900 advantageously achieves full-field optical transmission. Theconfiguration of transmitter 3900 thus represents a trade-off withrespect to the full-field optical transmission principles describedabove with respect to FIGS. 35 and 36, in that transmitter 3900 utilizesan external phase modulator (e.g., phase modulator 3914), but onlyrequires one slave laser (e.g., slave laser 3904), and avoids the needfor optical couplers and phase shifters. Additionally, transmitter 3900is not require bias control for phase modulator 3914, and because theinsertion loss of phase modulator 3914 is lower than the insertion lossusing an MZM (e.g., external modulator 3204, FIG. 32), a higher outputpower may be achieved over that using a conventional external I/Qmodulator.

FIG. 40 is a schematic illustration of an exemplary dual-polarizationCOIL transmitter 4000 implementing full-field modulation based onamplitude and phase modulations. More particularly, for a single masterlaser 4002, transmitter 4000 utilizes two amplitude/phase COILsubsystems 3906, FIG. 39, to modulate the respective amplitude and phasedata streams for each of an X-polarization and a Y-polarization. In theexample depicted in FIG. 40, the laser signal from master laser 1002 isdistributed to a first COIL subsystem 3906(X) and a second COILsubsystem 3906(Y) by a preliminary coupler 4004. First COIL subsystem3906(X) then synthesizes a single-polarization optical signal 4006(X)from a first amplitude-data source 4008(X) and a first phase-data source4010(X) of the X-polarization, and second COIL subsystem 3906(Y)synthesizes a single-polarization optical signal 4006(Y) from a secondamplitude-data source 4008(Y) and a second phase-data source 4010(Y) ofthe Y-polarization. Optical signals 4006(X), 4006(Y) are then combinedby a PBC 4012 to produce a dual-polarization modulated optical outputsignal 4014.

In the embodiments described above, networking of multipleinjection-locked transmitters is an important consideration. Theembodiments described above with respect to FIGS. 35-40 represent stillfurther advancements over conventional transmission techniques, in thatthese embodiments may be fully implemented with different network types,such as P2P or P2MP links. The present COIL techniques advantageouslyenable the multiple transmitters to operate either locally or remotely,and for both single-wavelength source and multi-wavelength sourceconfigurations, as described further below with respect to a FIGS.41-43. For ease of explanation, references to full-field transmitters inthe description of FIGS. 41-43 may include any one of the embodimentsdescribed above with respect to FIGS. 35-40, or a combination of morethan one of the embodiments described therein.

FIG. 41 is a schematic illustration of an exemplary optical network4100. In an exemplary embodiment, network 4100 includes an aggregationhub 4102 having a single-wavelength master laser source 4104 that isdistributed to a plurality (i.e., 1-N) of full-field COIL transmitters4106 through a power splitter 4108 configured to split the high-qualitysignal from master laser source 4104 for injection into the slave lasers(not shown in FIG. 41) of the respective full-field transmitters 4106.In an embodiment, each of full-field transmitters 4106 may operablyconnect to at least one different remote node 4110 over its ownrespective fiber segment 4112. According to the exemplary configurationof network 4100, single-wavelength master laser source 4104 is enabledto injection-lock multiple full-field transmitters 4106, whichsignificantly reduces the overall system cost.

FIG. 42 is a schematic illustration of an exemplary optical network4200. Network 4200 is similar to network 4100, FIG. 41, and includes anaggregation hub 4202 having a single-wavelength master laser source 4204that is distributed to a plurality (i.e., 1-N) of full-field COILtransmitters 4206 through a power splitter 4208 that splits thehigh-quality signal from master laser source 4204 for injection into therespective slave lasers of full-field transmitters 4206. Different fromnetwork 4100 though, network 4200 is implemented for a single-fiber P2Plink using a multi-core or multimode fiber 4210.

Accordingly, at the transmitter-side of fiber 4210, each of full-fieldtransmitters 4206 (i.e., including respective slave lasersinjection-locked by single-wavelength master laser 4204) is multiplexedonto fiber 4210 as a single aggregate optical signal by a multiplexer4212 for delivery to a receiver aggregation 4214, which includes aplurality (i.e. 1-N) of optical receivers 4216, through a demultiplexer4218. In some embodiments, multiplexer 4212 is a mode multiplexer, inwhich case fiber 4210 may be a multi-mode fiber, and demultiplexer 4218may be a mode demultiplexer. In other embodiments, multiplexer 4212 is acore multiplexer, in which case fiber 4210 may be a multi-core fiber,and demultiplexer 4218 may be a core demultiplexer.

FIG. 43 is a schematic illustration of an alternative optical network4300. Network 4300 is similar to network 4200, FIG. 42, and includes anaggregation hub 4302. Network 4300 differs from network 4200 though, inthat hub 4302 includes a multi-wavelength master laser source 4304(e.g., comb source) distributed to 1-N full-field COIL transmitters 4306through a WDM demultiplexer 4308 that separates the differentwavelengths of master laser source 4304 for injection into therespective slave lasers of full-field transmitters 4306. Network 4300 istherefore also particularly useful as a P2P single-fiber WDM networkconfigured to utilize a single optical transport medium 4310 (e.g., anoptical fiber).

Accordingly, at the transmitter-side of fiber 4310, each of full-fieldtransmitters 4306 are injection-locked to at least one wavelength of themulti-wavelength comb source of master laser source 4304, and thenmultiplexed onto fiber 4310 as an aggregate optical signal by a WDMmultiplexer 4312 for delivery to a receiver aggregation 4314 of 1-Noptical receivers 4316 through a WDM demultiplexer 4318. That is,according to the embodiment depicted in FIG. 43, each of full-fieldtransmitters 4306 may transmit on different wavelengths, multiplex thesedifferent wavelengths using WDM multiplexer 4312, and transmit theresultant aggregated signal over single fiber 4310, where it may bedemultiplexed by WDM demultiplexer 4318 prior to detection by therespective different receivers 4316.

According to the preceding embodiments, innovative systems and methodsare provided for injection-locking full-field transmitters that arefully enabled to incorporate new technologies that meet the uniquerequirements of the access network environment, but throughcost-efficient approaches. For example, using significantly low-costlasers, the present embodiments are able to simultaneously achieve bothlaser injection-locking and full-field signal modulation, such as m-QAMor m-PSK. Furthermore, as described above, full-field signal modulationmay be successfully achieved through either I/Q modulation oramplitude/phase modulation. Additionally, the present systems andmethods are fully adaptable from single-polarization configurations todual-polarization configurations without complex structuralmodifications to the transmitters or the networks in which they aredeployed.

The present systems and methods are still further fully scalable forsystem architectures that utilize multiple injection-lockedtransmitters, whether for single-wavelength source use cases or formulti-wavelength source use cases, and for both P2P and P2MP networks.According to the techniques described above, a single high-qualitymaster laser may be efficiently cloned and modulated by many low-costslave lasers, whereby these slave lasers will realize the samehigh-quality performance, and carry the same advanced modulationformats, of the high-quality source. Accordingly, in comparison withconventional networks, the overall cost of the present network isgreatly reduced by avoiding the need to deploy high-quality, high-costmaster lasers and modulators in each coherent transmitter/transceiver.

Phase Domain Signal Modulation and Equalization

As described above, coherent technologies are being increasinglydeployed for ultra-high-speed long-haul backbone transport systems/linksand metro networks due to recent DSP advancements, advanced signalformats, polarization multiplexing, and data rate/spectrum efficiencyimprovements. There is a clear desire in the industry to further thistrend in the access network environment, which is considered to be themost effective future-proof approach for optical access networks inbrown and green field deployments. This need is felt particularly in thecable access environment, where coherent optics technologies enableoperators to best leverage existing fiber infrastructures for thepresent exponential growth in capacity and services.

Introduction of digital coherent technologies into optical accessnetworks has presented several engineering challenges, because theaccess network environment is a significantly different environment thanthe long-haul and metro environments, particularly with respect to theoverall system cost that is dominated by optical and optic-electroniccomponents, such as low-linewidth tunable lasers source and advancedmodulators. The embodiments described above provide a number ofinnovative solutions for reducing the cost of lasers the access network,and also for simplifying the configuration of the various modulators.The embodiments described immediately above further provide novel phasemodulation techniques using COIL technologies to realize both full-fieldoptical transmitters, and which also further simplify the configurationand operation of the transmitters. These solutions are particularlyadvantageous over conventional MZM techniques for signal modulation,even with respect to some of the techniques described herein thatimplement I/Q signal modulation.

Such advantages include without limitation: (1) the total loss that isseen according to the present phase modulation techniques issignificantly smaller than the loss experienced using a conventional MZM(i.e., >3 dB), or some I/Q modulation techniques (e.g., >6 dB), therebygreatly improving the present output power; (2) the cost of transmitterimplementing the present phase modulator embodiments is expected to besignificantly lower than the cost of implementing an MZM, since one MZMtypically includes two phase modulators, and an also lower than the costof implementing an I/Q modulator, since one I/Q modulator may include asmany as phase modulators; (3) since the signal according to the presentembodiments may be modulated in phase domain, the amplitude andintensity of the output signal, after phase modulation, will remain thesame; and (4) the modulated signal, after passing through the presentphase modulator, will achieve better performance under fibernonlinearities.

Nevertheless, one particular challenge to implementation of phase domainsignal modulation arises with respect to signal equalization.Conventional coherent DSP algorithms have not been able to successfullyovercome phase domain inter-symbol-phase-interference in thephase-modulated signals. This interference problem though, is solvedaccording to the following innovative systems, apparatuses, and methodsfor multi-level signal modulation and equalization in the phase domainfor optical coherent communication systems. The present embodiments thusprovide a complete end-to-end system configuration for signalmodulation, coherent detection, and equalization for opticallytransmitted multi-level signals.

In an exemplary embodiment, a multi-level signal is mapped into thephase domain, and then modulated by a phase modulator on a particularpolarization. Once modulated, the optical signal may then be coherentlydetected and processed by DSP to recovery. The following techniques forphase domain equalization to address inter-symbol-phase-interface may beadvantageously implemented at either the transmitter-side or thereceiver-side, whether as post-equalization and/or pre-equalization.More particularly, in some embodiments, post-equalization techniques areprovided herein for receiver-side processing in the phase domain. Inother embodiments, digital phase-domain pre-compensation techniques areprovided for improving performance at the transmitter-side.

The following embodiments are therefore particularly useful to satisfythe unique requirements of the access environment, by providingarchitectural configurations of significantly lower complexity, and alsoby realizing a significantly reduced insertion loss and modulation lossfor multi-level signals, in comparison with MZM- and I/Q modulator-basedsystems. As described further below, the sensitivity performance ofmulti-level signals modulated according to the present phase modulationembodiments is greatly enhanced in comparison with multi-level signalsmodulated in the amplitude or intensity domains.

FIG. 44 is a schematic illustration of an exemplary optical coherentcommunication system 4400 for modulating, detecting, and equalizingcoherent signals in the phase domain. In an embodiment, system 4400includes a transmitter-side 4402 in operable communication with areceiver-side 4404 over an optical fiber network 4406. In an exemplaryembodiment, transmitter-side 4402 is configured to modulate multi-levelsignals 4408 (e.g., PAM-N electrical signals) onto the phase of anoptical carrier using an optical phase modulator 4410. A resultingoptically modulated signal 4412, after transmission over optical fibernetwork 4406, may then be coherently detected by an integrated coherentreceiver (ICR) 4414 in communication with an LO 4416 and receiver-side4404. The coherently-detected signal from ICR 4414 is then subjected toconversion by an ADC 4418, and then equalized and demodulated by areceiver-side DSP 4420 into demodulated multi-level signals 4422.According to the present techniques, by modulating, detecting, andequalizing coherent optical signals in the phase domain, system 4400provides a significant improvement over conventional multi-level opticalcommunication systems that modulate multi-level signals in the amplitudeor intensity domain.

FIG. 45 is a graphical illustration depicting an operational principle aphase domain mapping module 4500 that may be implemented with one ormore of the embodiments described herein to map a multi-level signal4502 into a phase domain signal 4504. In some embodiments, module 4500may represent a discrete hardware unit (e.g., having a processor and amemory). In other embodiments, module 4500 may be implemented throughsoftware programming or computer-executable instructions that may beexecuted by a processor of a relevant transmitter, receiver,transceiver, or other element of the particular communication network orsystem. In at least one embodiment, module 4500 is implemented throughcombination of dedicated hardware and software. For ease ofillustration, and not in a limiting sense, the following examples aredescribed with respect to multi-level signal 4502 being a 4-level PAM-4signal. The person of ordinary skill in the art will understand,however, that other multi-level signals may be used with respect to thepresent embodiments, but without departing from the scope thereof.

In exemplary operation of phase domain mapping module 4500, input bitinformation (e.g., [1110000110 . . . ]) coded as PAM-4 symbols ofmulti-level signal 4502 by a symbol coding unit 4506. Once encoded,Then, the PAM-4 symbols of multi-level signal 4502 may be mapped tospecific phase values of phase domain signal 4504 by a phase domainsignal mapping unit 4508. In the exemplary embodiment depicted in FIG.45, the PAM-4 symbols of signal 4502 are mapped as phases of [+3pi/4,+pi/4, −pi/4, −3pi/4]. Accordingly, the relevant optical carrier in thisexample will be modulated as a corresponding 4PSK (QPSK) signal. Thatis, the drive signal for phase modulation will have four levels, whichis mapped as four phases of 4PSK optical signal 4504.

In a similar manner, the operational principle of module 4500 may beeffectively applied for any N-level signal. For example, after PAM-Nmapping or coding in the electrical domain of an N-level signal 4502_(N) by symbol coding unit 4506, the optical carrier may be modulated asan N-PSK signal 4504 _(N) by phase domain signal mapping unit 4508. Thatis, the corresponding drive signal for phase modulation by phase domainsignal mapping unit 4508 will have N levels, and therefore be mapped asN phases of N-PSK optical signal 4504 _(N). Module 4500 may then beoperable for a respective phase modulator on the transmitter-side (e.g.,phase modulator 4410, FIG. 44) or demodulator on the receiver-side(e.g., DSP 4420, FIG. 44).

FIG. 46A is a schematic illustration of an exemplary single-polarizationtransmitter 4600 for multi-level signal modulation in the phase domain.More particularly, the embodiment depicted in FIG. 46A illustrates aprocess flow for transmitter 4600 implementing phase domain mappingmodule 4500, FIG. 45, with an external phase modulation subsystem 4602.In an embodiment, subsystem 4602 includes a laser 4604 and a phasemodulator 4606 configured to emit a phase domain modulated light-waveoptical signal 4608. In some embodiments, laser 4604 and phase modulator4606 are separate and discrete components from one another. In otherembodiments, laser 4604 and phase modulator 4606 are integrated togetheras unitary phase-modulated laser.

In exemplary operation of transmitter 4600, input data is first encodedinto multi-level signals (e.g., PAM-N signals 4502 _(N)) by multi-levelsignal coding unit 4506, and then mapped to corresponding phase valuesof optical signals (e.g., signals 4504 _(N)) in the phase domain byphase domain signal mapping unit 4508. As described above, the drivesignal for phase modulation will have N levels, which is mapped as Nphases of the resulting N-PSK optical signal 4504 _(N). In practicalimplementation, the drive voltage may be determined by the voltage Vπ ofphase modulator 4606, where Vπ is the required voltage for a π phasechange of phase modulator 4606. Accordingly, in some embodiments,transmitter 4600 may optionally further include at least one electricalamplifier 4610 to boost the drive voltage of N-level signal 4504 _(N) tomeet the voltage requirement of Vπ.

FIG. 46B is a schematic illustration of an alternativesingle-polarization COIL transmitter 4612. In the embodiment depicted inFIG. 46B, transmitter 4612 implements phase domain mapping module 4500,FIG. 45, with a COIL phase modulation subsystem 4614, and may furtherinclude an optional amplifier 4616. In an exemplary embodiment,subsystem 4612 is similar to COIL subsystem 3906, FIG. 39, and isconfigured to receive a laser signal from an external master lasersource 4618, and also includes an optical circulator 4620, a phasemodulator 4622, and a slave laser 4624. Similar to COIL subsystem 3906,optical circulator 4620 routes the laser signal from master laser 4618into a resonance cavity of injection-locked slave laser 4624, and whichthen also routes the resulting optical signal from slave laser 4624,which is externally modulated by phase modulator 4622 with the generatedmulti-level optical signals from module 4500.

In some use cases of amplifier 4600 or amplifier 4612, it may beadditionally desirable to implement further DSP processing at thereceiver-side (e.g., DSP 4420, FIG. 44), to address phase-domaininter-symbol-phase-interference (ISPI) that may result from a non-idealfrequency response of the respective transmitter (e.g., from limitedbandwidth, or frequency fading of the signal generator and phasemodulator thereof). Conventional coherent optical system DSP techniques,however, are incapable of resolving such distortion effects. Aninnovative receiver-side DSP solution though, for resolving ISPI, isdescribed further below with respect to FIG. 47.

FIG. 47 is a schematic illustration of a phase domain equalizationprocess 4700 performed at the receiver-side (e.g., receiver side 4404,FIG. 44) for post-equalization. In an exemplary embodiment, process 4700is executed by a DSP (e.g., DSP 4420), which receives signal data 4702from a coherent receiver and an ADC (e.g., ICR 4414 and ADC 4418, FIG.44), and further processes signal data 4702 using one or more of a CDcompensation unit 4704 a clock recovery unit 4706, a polarizationrecovery and channel equalization unit 4708, a frequency offsetestimation unit 4710, and a phase domain noise estimation andequalization unit 4712, and then outputs modulated multi-level signals4714.

In the exemplary embodiment, phase recovery is performed primarily atthe end of processing, namely, by phase domain noise estimation andequalization unit 4712, after preliminary CD compensation, clockrecovery, channel equalization, and/or frequency-offset estimationprocessing may have been performed. More specifically, unit 4712implements process 4700 as into steps: (1) a phase recovery step S4716;and (2) a phase domain equalization step S4718.

In step S4716, phase recovery is performed to demodulate the multi-levelsignal. More particularly, phase recovery step S4716 includes a substepS4720, in which the dynamic phase noise caused by laser linewidth isestimated as φ_(n)(t). In substep S4722 the estimated dynamic phasenoise value is removed by multiplying (e.g., by a multiplication unit)by the inverse value as exp[−jφ_(n)(t)]. In some embodiments of process4700, the phase noise estimation of substep S4720 may be realizedaccording through use of conventional phase noise estimation algorithms,such as training-based phase estimation algorithms, Viterbi-viterbialgorithms, and blind phase search (BPS) algorithms.

After the dynamic phase noise value is removed, phase domainequalization step S4718 of process 4700 is enabled to execute a mappingsubstep S4724, in which the signal optical phase may be mapped back tomulti-level signals, and in substep S4726, a time-domain adaptiveequalization is applied to the obtained PAM-N signal (in this example),Which is then equalized by a K-tap finite impulse response (FIR) digitalfilter to output the modulated multi-level signals 4714. In step S4728,a tap value may be updated by an error function after equalization. Insome embodiments of step S4728, updating may be performed using suchcommon adaptive equalization algorithms as least-mean-square (LMS), andrecursive least squares (RLS) for filter tap value updating. In analternative embodiment of step S4726, other adaptive equalizationconfigurations be implemented, such as a Decision-Feedback Equalizer(DFE) for PAM-N signal equalization. In at least one embodiment, inorder to hasten the convergence and tracking speed of the adaptiveequalizer, a training sequence may be used for error functioncalculation.

In other cases, it may be desirable to perform pre-equalization at thetransmitter-side (e.g., transmitter-side 4402, FIG. 44). An exemplarysolution for such transmitter-based pre-equalization techniques isdescribed below with respect to FIG. 48.

FIG. 48 is a schematic illustration of a phase domain modulatedtransmitter 4800 configured to implement pre-equalization at thetransmitter-side (e.g., transmitter-side 4402, FIG. 44) forpre-equalization prior to phase modulation. That is, in an embodiment,transmitter 4800 is substantially similar to transmitter 4600, FIG. 46A,and similarly illustrates a process flow for transmitter 4800 having anexternal phase modulation subsystem 4802 implementing a phase domainmapping module 4804, and an external amplifier 4806 disposedtherebetween. Similar to external phase modulation subsystem 4602,subsystem 4802 includes a laser 4808 and a separate or integral phasemodulator 4810 configured to emit a phase domain modulated light-waveoptical signal 4812.

Exemplary operation of transmitter 4800, is therefore substantiallyperformed in accordance with the operation of transmitter 4600, FIG. 46.However, phase domain mapping module 4804 is different from phase domainmapping module 4500, in that module 4804 includes an additionalpre-equalization unit 4818 disposed between an output of phase domainsignal mapping unit 4816, and before communication with phase modulator4810. That is, in operation of transmitter 4800, the phase-domainchannel response may be obtained at the receiver-side (e.g., receiverside 4404, FIG. 44), since the channel response is generally consideredto be quasi-static for the relevant optical channel, it may also beapplied at the transmitter-side (e.g., transmitter-side 4402, FIG. 44).

Transmitter 4800 therefore represents an exemplary implementation oftransmitter 4600, FIG. 46A, but with enhanced capabilities forpre-equalization in phase domain mapping module 4804 through utilizationof pre-equalization unit 4818. That is, pre-equalization unit 4818functions to add a pre-equalization process after phase domain signalmapping by phase domain signal mapping unit 4816. Through thisadvantageous configuration, the relevant signal transmitter-sidepre-equalization may be accomplished in either the time-domain or in thefrequency-domain, and the relevant receiver-side response may then beobtained through the post-phase domain equalization techniques describedabove with respect to FIG. 47. For example, the K-tap filter of stepS4726, FIG. 47, after convergence, may also be used for transmitter-sidepre-equalization. Because the channel response may also be estimated bythe K-tap filter, the frequency-domain pre-equalization may thereforealso be applied based on the frequency response of the receiver-sideK-tap filter.

FIG. 49 is a flow chart diagram of an exemplary pre-equalization process4900 based on receiver-side channel estimation. In an exemplaryembodiment, process 4900 represents an exemplary technique forimplementing pre-equalization at the transmitter-side (e.g. transmitterside 4402, FIG. 44) using channel estimates on the receiver-side (e.g.,receiver-side 4404, FIG. 44), and is fully adaptable to one or more ofthe embodiments described herein. Except where described to thecontrary, individual steps of process 4900 may be performed in the orderdescribed, a different order, and/or two or more of the several stepsmay be performed simultaneously.

Process 4900 begins at step 4902, in which a multi-level signal isgenerated without pre-equalization, and then is modulated onto anoptical phase. In an exemplary embodiment of step 4902, the generatedmulti-level signal is further mapped into the phase domain withoutpre-equalization and modulated onto the phase of optical carrier. Instep 4904, the relevant receiver coherently detects the signal andperforms DSP, including application of phase domain post-equalization.In step 4906, a pre-equalization filter or frequency response may bederived based on the K-tap adaptive filter of the phase-domain postequalization (e.g., in the receiver DSP). In step 4908, the receivercommunicates the derived pre-equalization filter or frequency responseback to the transmitter to perform pre-equalization. In step 4910, thetransmitter generates the multi-level signal with pre-equalizationaccording to the techniques described above, and modulates thepre-equalized multi-level signal onto the optical phase.

Process 4900 may thus be equally implemented with respect to the generaltransmitter configuration depicted in FIG. 46A, or for the particularCOIL subsystem-based transmitter configuration depicted in FIGS. 46B and48. It may be further noted here that even after the transmitter hasperformed phase-domain pre-equalization, residual ISPI may still bepresent in system. In such circumstances, the present phase domainpost-equalization techniques may be performed together withpre-equalization, added as an extra step or an extra unit, to one orboth of modules 4500, FIG. 45, and 4804, FIG. 48

Through various real-world testing simulations proof of theabove-described concepts was verified by the present inventors. In onetest example, the performance of a 12.5 GBaud polarization-multiplexedPAM-4 signal was evaluated, since the PAM-4 signal may be deemedequivalent to a 12.5 GBaud polarization multiplexed QPSK signal afterphase domain modulation. Verification of these principles, includingphase domain multilevel signal modulation and post-equalization withcoherent detection, is described below with respect to FIGS. 50A-F.

FIGS. 50A-F are graphical illustrations depicting signal results 5000,5002, 5004, 5006, 5008, 5010, respectively, of receiver-side DSP (e.g.,receiver-side DSP 4420, FIG. 44). More particularly, FIGS. 50A-F depictresults of a modulated optical signal at several different stages ofreceiver-side DSP processing. That is, FIG. 50A depicts a phase plot5000 of the optical signal after frequency-offset estimation (e.g., byfrequency offset estimation unit 4710, FIG. 47). FIG. 50B depicts a plot5002 of the estimated optical signal phase noise φ_(n)(t) (e.g., byphase domain noise estimation and equalization unit 4712). FIG. 50Cdepicts a QPSK signal constellation 5004 without phase domainequalization. FIG. 50D depicts a corresponding phase counterpart 5006 ofQPSK signal constellation 5004, FIG. 50C, mapped back to PAM-4 signals(e.g., in step S4724 of unit 4712). FIG. 50E depicts equalized PAM-4signals 5008 after phase domain post-equalization (e.g., by unit 4712).FIG. 50F depicts a QPSK signal constellation 5010 after phase domainequalization has been performed.

Accordingly, despite the overt existence of ISPI in the case of largephase fluctuations in four QPSK phases, considerable improvements maynevertheless be readily seen through a simple comparison of therespective constellations of FIGS. 50C and 50F, and also through asimple comparison of FIGS. 50D and 50E.

FIG. 51 is a graphical illustration depicting a comparative plot 5100 ofBER against received optical power. In an exemplary embodiment, plot5100 represents a comparison of the BER against received power for afirst sub-plot 5102 representing a PAM-4 (QPSK) signal after coherentdetection, and with phase domain equalization, and for a second sub-plot5104 representing a PAM-4 (QPSK) signal after coherent detection, butwithout phase domain equalization. As can be seen from the exampledepicted in FIG. 51, significant performance improvements areobservable. For example, it may be seen that the required optical powermay realize improvements of 3.5 dB at a BER value of 1E−3, greater than3 dB at a BER value of 4.5E−3, and still greater than 2 dB at a BERthreshold of 1E−2.

According to the innovative systems and methods provided herein, uniquesystems, apparatuses, and methods are provided for multi-level signalmodulation and equalization in the phase domain for optical coherentcommunication networks, and particularly useful in the access networkenvironment. The present embodiments further provide end-to-end systemconfigurations for signal modulation, coherent detection, andequalization for multi-level signal transmissions. In exemplaryembodiments, the multi-level signals are mapped into phase domain, andthen modulated by phase modulator on a single polarization. Theresulting optical signals may then be coherently detected, and thenfurther processed by a DSP to recovery.

The present systems and methods further provide two individual types ofphase-domain equalization methods for resolving theinter-symbol-phase-interface that plagues conventional phase modulationproposals for coherent optical access networks. The present techniquesprovide for both post-equalization at the receiver-side, and alsopre-equalization at the transmitter-side. As further described above,these unique solutions provide yet a third, hybrid solution thatuniquely implements a synergistic combination of both of thepost-equalization and pre-equalization techniques described herein. Thatis, a unique post-equalization process is implemented in thereceiver-side DSP to resolve ISPI problems downstream, and a digitalphase-domain pre-compensation process may also be implemented to realizesignificant performance improvements at the transmitter-side.

Exemplary embodiments of optical communication systems and methods aredescribed above in detail. The systems and methods of this disclosurethough, are not limited to only the specific embodiments describedherein, but rather, the components and/or steps of their implementationmay be utilized independently and separately from other componentsand/or steps described herein. Additionally, the exemplary embodimentscan be implemented and utilized in connection with other access networksutilizing fiber and coaxial transmission at the end user stage.

As described above, the DOCSIS protocol may be substituted with, orfurther include protocols such as EPON, RFoG, GPON, Satellite InternetProtocol, without departing from the scope of the embodiments herein.The present embodiments are therefore particularly useful forcommunication systems implementing a DOCSIS protocol, and may beadvantageously configured for use in existing 4G and 5G networks, andalso for new radio and future generation network implementations.

Although specific features of various embodiments of the disclosure maybe shown in some drawings and not in others, such illustrativetechniques are for convenience only. In accordance with the principlesof the disclosure, a particular feature shown in a drawing may bereferenced and/or claimed in combination with features of the otherdrawings.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor or controller, suchas a general purpose central processing unit (CPU), a graphicsprocessing unit (GPU), a microcontroller, a reduced instruction setcomputer (RISC) processor, an application specific integrated circuit(ASIC), a programmable logic circuit (PLC), a field programmable gatearray (FPGA), a DSP device, and/or any other circuit or processorcapable of executing the functions described herein. The processesdescribed herein may be encoded as executable instructions embodied in acomputer readable medium, including, without limitation, a storagedevice and/or a memory device. Such instructions, when executed by aprocessor, cause the processor to perform at least a portion of themethods described herein. The above examples are exemplary only, andthus are not intended to limit in any way the definition and/or meaningof the term “processor.”

This written description uses examples to disclose the embodiments,including the best mode, and also enables a person skilled in the art topractice the embodiments, including the make and use of any devices orsystems and the performance of any incorporated methods. The patentablescope of the disclosure is defined by the claims, and may include otherexamples that occur to those skilled in the art. Such other examples areintended to be within the scope of the claims if they have structuralelements that do not differ from the literal language of the claims, orif they include equivalent structural elements with insubstantialdifferences from the literal language of the claims.

What is claimed is:
 1. An optical injection locking based coherentoptical transmitter for a coherent optical communications network, thecoherent optical transmitter comprising: a master laser sourceconfigured to provide a low linewidth frequency channel as a masterlaser signal; a first coherent optical injection locking (COIL)subsystem including (i) a first slave laser configured for COIL with amaster frequency of the master laser signal, (ii) an optical circulatorconfigured to inject the master laser signal into a cavity of the firstslave laser, and (iii) a full-field modulator configured to output afirst modulated optical signal based on an output of the first slavelaser routed through the optical circulator.
 2. The transmitter of claim1, wherein the first slave laser is configured to receive electricalin-phase data, and wherein the first COIL subsystem further includes asecond slave laser configured to receive electrical quadrature data. 3.The transmitter of claim 2, wherein the first COIL subsystem furtherincludes a first optical coupler disposed between (i) the opticalcirculator and the first slave laser, and (ii) the optical circulatorand the second slave laser.
 4. The transmitter of claim 3, wherein thefirst COIL subsystem further includes a first phase shifter disposedbetween the first optical coupler and the second slave laser.
 5. Thetransmitter of claim 4, wherein the first phase shifter is configured toimplement a 45-degree one-way phase shift.
 6. The transmitter of claim4, wherein the first COIL subsystem further includes a second phaseshifter.
 7. The transmitter of claim 6, wherein the first COIL subsystemfurther includes a second optical coupler disposed between the masterlaser source and the optical circulator and a third optical couplerdisposed between the optical circulator and the first modulated opticalsignal, and wherein the second phase shifter is disposed between thesecond and third optical couplers.
 8. The transmitter of claim 6,wherein the first COIL subsystem further includes a third slave laserconfigured for DC offset cancelation, and wherein the second phaseshifter is disposed between the first optical coupler and the thirdslave laser.
 9. The transmitter of claim 1, wherein the full-fieldmodulator comprises a phase modulator disposed between the opticalcirculator and the first modulated optical signal.
 10. The transmitterof claim 9, wherein the first slave laser is configured to receiveelectrical amplitude data, and wherein the phase modulator is configuredto receive electrical phase data.
 11. The transmitter of claim 1,further comprising a second COIL subsystem configured to output a secondmodulated optical signal, and a polarization beam combiner configured tocombine the first modulated optical signal with the second modulatedoptical signal.
 12. The transmitter of claim 11, wherein the first COILsubsystem is configured to output the first modulated optical signal forX-polarization data, and wherein the second COIL subsystem is configuredto output the second modulated optical signal for Y-polarization data.13. An optical communication network, comprising: an optical hubincluding: a master laser source; and a plurality of full-field coherentoptical transmitters disposed proximate the optical hub, wherein eachfull-field transmitter of the plurality of full-field coherent opticaltransmitters (i) is configured to transmit a downstream coherent opticalsignal using a center frequency of a master signal from the master lasersource, and (ii) includes a first slave laser injection locked to thecenter frequency of the master signal; a receiver; and an opticaltransport medium operably connecting the plurality of full-fieldcoherent optical transmitters to the receiver.
 14. The network of claim13, wherein the optical hub further includes a power splitter operablyconnecting each full-field transmitter of the plurality of full-fieldcoherent optical transmitters to the master laser source.
 15. Thenetwork of claim 14, wherein the receiver comprises a plurality of fibernodes, and wherein the optical transport medium comprises a plurality ofoptical fibers connecting the plurality of fiber nodes to a respectivefull-field transmitter of the plurality of full-field coherent opticaltransmitters.
 16. The network of claim 15, wherein the network isconfigured for point-to-multipoint operation.
 17. The network of claim14, wherein the receiver comprises a plurality of receiving units,wherein the optical transport medium comprises a multi-mode fiber,wherein the plurality of full-field coherent optical transmittersoperably connect to the multi-mode fiber through a mode multiplexer, andwherein the plurality of receiving units operably connect to themulti-mode fiber through a mode demultiplexer.
 18. The network of claim14, wherein the receiver comprises a plurality of receiving units,wherein the optical transport medium comprises a multi-core fiber,wherein the plurality of full-field coherent optical transmittersoperably connect to the multi-core fiber through a core multiplexer, andwherein the plurality of receiving units operably connect to themulti-core fiber through a core demultiplexer.
 19. The network of claim13, wherein the master laser source is a master comb source, and whereinthe optical hub further includes a firstwavelength-division-multiplexing (WDM) demultiplexer operably connectingeach full-field transmitter of the plurality of full-field coherentoptical transmitters to the master comb source.
 20. The network of claim19, wherein the receiver comprises a plurality of receiving units,wherein the optical transport medium comprises an optical fiber, whereinthe plurality of full-field coherent optical transmitters operablyconnect to the optical fiber through a WDM multiplexer, and wherein theplurality of receiving units operably connect to the optical fiberthrough a second WDM demultiplexer.